Big-game hunters will find it far more difficult and less lucrative to donate their extra trophy mounts and claim charitable tax deductions under new tax rules being debated this week on the Senate floor.

Tightening the trophy-mount tax break, and making sure that museums do not accept donated items with the intention of quickly selling them off, have been identified as priorities by Sen. Charles E. Grassley (R-Iowa), chairman of the Senate Finance Committee.

New language that would substantially close the loopholes was voted out of the Finance Committee on Tuesday. Grassley said yesterday that he expects Senate approval and that "I hope my House colleagues will see it's only fair to taxpayers to rein in trophy abuse."

The loophole Grassley said he is seeking to close allows big-game hunters to deduct some or all of the cost of their safaris if they later donate to a museum some of the trophy animals they kill and have mounted. The new rules would allow some donations, but would limit the amount of charitable contribution that could be deducted to the market value of the specimen, rather than the replacement value.

In addition, the new provisions would make it far more difficult for museums to accept hundreds of donations that they had no intention of keeping and displaying to the public. The Wyobraska Wildlife Museum in Gering, Neb., for instance, received hundreds of donated trophy mounts in recent years and kept most of them in trailers behind its modest facility before selling them at a taxidermy auction for a small percentage of the value claimed by donors.

Grassley said tightening the trophy mount loophole would save several million dollars over 10 years, and ensure that museums accept only items they plan to keep in their collections. The moves would save as much as $40 million over that period.

"That shows the experts believe a good number of hunters are exploiting the loophole to fund their trips to exotic locations," Grassley said.

"The equivalent for non-hunters would be if someone bought a sweater in Paris, donated it to Goodwill, and took a tax deduction for the entire trip to Paris," he said. "The tax code should encourage legitimate donations, but only legitimate donations."

The practice of donating hunting trophies has been broadly advertised within the big-game hunting community, with one appraiser from Chicago sending out brochures promoting ways to "Hunt for Free." Hunters have also been encouraged to set up "museums" in their homes that would allow for large tax write-offs.

The Senate language was welcomed yesterday by the Humane Society of the United States, which has sought to focus attention on the existence of hunting parks where exotic animals are raised and hunted for a fee. Some of the questionable trophy mounts donated to museums were killed in such parks, especially in several larges ones in Texas.

"This is an excellent fix to the problem" of inflated claims for animal donations, said Michael Markarian, the group's executive vice president. "This is very similar to what Congress did with vehicle donations last year and will prevent the overwhelmingly majority of these abuses."

Calls to several groups that promote hunting interests were not returned yesterday, but the organizations have numerous friends in Congress. A Republican Finance Committee staff member said he was unaware of any opposition to the provision in the Senate, but that it remained unclear whether it would be accepted by the House in a conference committee.

Sen. Charles E. Grassley (R-Iowa) in his office with an example of the big-game trophy mounts at issue in the debate over charitable tax deductions for hunters.