When this break was agreed upon by President Obama and the Republican Congress at the end of 2010, I expected it would?spur the wealthiestto give away huge amounts of their estate to take advantage of the break. (The original $5 million limit was increased this year for inflation; it is separate from the $13,000 annual exclusion gift.) But the rush that I expected was?initially just a trickle. While the tax break was enticing, the fear of what lay ahead in the economy and financial markets made people cautious.

Now, some of the wealthy are faced with a choice that seems designed by a behavioral economist to test rational decision-making: Do they give their heirs the full amount of the exemption, happy that the money will help the heirs now and reduce their eventual?estate tax?bill? Or do they give less, or none at all, for fear that they could be left with not enough to live on?

?I?ve gone through this with lots of people, and based on the reactions, no one whose net worth is in the nine figures has worried about having enough,? said Edward F. Koren, chairman of private wealth services at the Holland & Knight law firm. ?Below that, it depends on their spending levels and their values. That?s so personal to the client.?

The deadline for making a decision is fast approaching because advisers say it takes at least three months to do everything that needs to be done to set up a gift of this size.

All advisers agree that every financial decision is about more than saving on taxes. But in this instance, the math on both taxes and compound interest is compelling, particularly for younger people with wealth who have decades to live.

While none of the advisers would make a prediction on what might happen, no one thought the exemption would remain this high or the tax this low in 2013. After years of a $1 million gift exemption, the advisers agreed that this was a once-in-a-lifetime opportunity.

Adding to the benefit of the large tax exemption is the amount the gifted money can grow. For a child who received $5 million today in a trust that was invested broadly, that gift could grow in 30 years to nearly $29 million, at a 6 percent return every year, according to calculations by Jonathan Blattmachr, a principal of Eagle River Advisors, which consults on estate planning. Meanwhile, the parents? estate would have been reduced by $29 million, cutting the tax due on the estate.

And that is why some wealthy people ? those who could run out of money in their lifetime ? have been seduced by this opportunity but are, at the same time, trying to work out how to do it. (The exemption for the estate tax is also $5.12 million until the end of the year, but, obviously, only those who die in 2012 can take advantage of it.)

Some people are also questioning the effect such a large gift will have on their heirs. Their main concern is that it will it rob their children of motivation, said Catherine McDermott, senior wealth planning strategist for Wells Fargo Private Bank.

But she said that fear may be misplaced. ?A lot of the education around stewardship of wealth occurs during a lifetime as you?re raising children,? she said. ?Many of those questions parents face as they?re raising children.?

In other words, by the time you have made enough money to think about leaving a lot of it to your heirs, you have either set them on the right path in life or led them to believe that your money will be their cushion.

For those who do decide to make a substantial gift, there are many different ways to do it. Writing a check is the simplest way, but advisers would tell you that would leave the money unprotected against creditors. It would also waste an opportunity to use various strategies to multiply the gift.

Putting appreciated securities in a trust would seem to be a good idea, but that could lock up liquid assets that might be needed.

Another option for people worried about having enough liquid assets is to put real estate or a share in a private business into a trust.

?Transferring a home doesn?t feel the same as transferring $5 million in stocks or bonds,? said Diane E. Lederman, president and chief executive of the Neuberger Berman Trust Company.

She cautioned that anyone doing this should make sure that all the paperwork was perfect because the people making the gift would essentially be renting their home back from a trust in their heirs? names. ?When you?re giving an asset away and you?re continuing to reside there, it?s going to attract I.R.S. scrutiny,? she said.

But those who worry that they may need the money back should understand that a gift that satisfies the?Internal Revenue Service?has to be irrevocable, and the person making that gift can no longer have control over the assets.

That would seem to be straightforward, but the world of tax planning is anything but. Mr. Blattmachr, who built his own wealth interpreting tax law for his clients at the law firm Milbank Tweed, where he was a partner for 35 years, has structured trusts with his wife, Betsy, that are so complex they boggle the mind. They enable the Blattmachrs to take advantage of the current exemptions and gift money today to reduce future estate taxes. But he has structured the trusts, which each hold about $4 million, in such a way that they have a built-in safety valve if something goes wrong: they can get the money back.

One risk to trusts like these is that they mirror each other and could then get disallowed by the I.R.S.

Another risk would be if the Blattmachrs divorced, but after 42 years of marriage, he said he was not worried about that. Many estate planners are wary of these trusts. ?There is very little out there from the I.R.S. on this,? said Paige Ben-Yaakov, a partner in the tax section at the law firm Baker Botts. ?It?s not as easy as taking the assets and giving them away because you know that works.?

Mr. Blattmachr has put great effort into making sure his trusts will be allowed. At the beginning of 2011, he began setting up his trust, naming his children, grandchild and his wife as beneficiaries. Earlier this year, he set up a trust for his wife that named their children, grandchild and him as beneficiaries. Setting up trusts at different times is one way to counter any future allegations by the I.R.S. that the gifts were not real and the trusts were reciprocal.

But Mr. Blattmachr went further. He put their two homes in his trust ? and now lives in them as a guest of his wife ? and put securities in her trust. He made sure the trusts had different sets of trustees, giving her the right to replace hers but denying himself that ability. Her trustees can convert her trust to pay her 4 percent a year regardless of the principal; his can make distributions only for health, maintenance and support.

?The I.R.S. always wants the low-hanging fruit,? Mr. Blattmachr said. ?You want to get yourself way, way up in the tree.?

He added, ?I don?t think any one can tell you that this absolutely works or this absolutely doesn?t work. But what they?re going to tell you is the I.R.S. will go after others before you.?