Inheritances are generally taxed at graduated rates depending on their value. In this case, the $29.2 million assessment for “Canyon” was based on a special penalty rate because the I.R.S. contends the heirs inaccurately stated its value.

While art lovers may appreciate the I.R.S.’s aesthetic sensibilities, some estate planners, tax lawyers and collectors are alarmed at the agency’s position, arguing that the case could upend the standard practice of valuing assets according to their sale in a normal market. I.R.S. guidelines say that in figuring an item’s fair market value, taxpayers should “include any restrictions, understandings, or covenants limiting the use or disposition of the property.”

In this instance, the 1940 Bald and Golden Eagle Protection Act and the 1918 Migratory Bird Treaty Act make it a crime to possess, sell, purchase, barter, transport, import or export any bald eagle — alive or dead. Indeed, the only reason Mrs. Sonnabend was able to hold onto “Canyon,” Mr. Lerner said, was due to an informal nod from the United States Fish and Wildlife Service in 1981.

Even then, the government revisited the issue in 1998. Rauschenberg himself had to send a notarized statement attesting that the eagle had been killed and stuffed by one of Teddy Roosevelt’s Rough Riders long before the 1940 law went into effect. Mrs. Sonnabend was then able to retain ownership as long as the work continued to be exhibited at a public museum. The piece is on a long-term loan to the Metropolitan Museum of Art in New York, which Mr. Lerner said insures it, but the policy details are confidential.

Mr. Lerner said that the I.R.S.’s handling of the work has been confusing. Last fall, the agency sent the family an unsigned draft report that it was valuing “Canyon” at $15 million. After Mr. Lerner replied that the children were refusing to pay, the I.R.S. then sent a formal Notice of Deficiency in October saying it had increased the valuation to $65 million.

That figure came from the agency’s Art Advisory Panel, which is made up of experts and dealers and meets a few times a year to advise the I.R.S.’s Art Appraisal Services unit. One of its members is Stephanie Barron, the senior curator of 20th-century art at the Los Angeles County Museum of Art, where “Canyon” was exhibited for two years. She said that the group evaluated “Canyon” solely on its artistic value, without reference to any accompanying restrictions or laws.