The Senate approved the first change yesterday in its increasingly sacrosanct tax-overhaul legislation -- to tighten the bill by removing a tax break for Unocal Corp. of California.

The amendment, sponsored by Sen. Howard M. Metzenbaum (D-Ohio), passed 60 to 33 with the support of the bill's sponsors. It was the first revision approved during the six days the tax package has been debated by the Senate.

Senate Finance Committee Chairman Bob Packwood (R-Ore.) and others said, however, that they do not expect the vote to lead to passage of amendments that would undermine the bill. Because it was part of an addition to the "transition rules" -- designed to ease the effect of the new law on certain companies, individuals and projects -- the amendment will not set a precedent, they said.

"We are clearly over the hump on the major amendments. I don't think the transitional rules are major," Packwood said.

"I don't see it as a signal a whole lot of other amendments will be adopted," said Sen. David L. Boren (D-Okla.).

The accuracy of those assessments will not be known until Monday, when the Senate is to resume voting on Metzenbaum's transition-rule amendments. Packwood said he hopes to bring the bill, a giant package of rate cuts that would limit numerous deductions, to a final vote by Tuesday.

He will face at least one more major amendment then, a proposal by Sen. George J. Mitchell (D-Maine) to reduce the bill's bottom personal tax rate from 15 percent to 14 percent, maintain what is now the bill's top rate of 27 percent and add a third rate of 35 percent.

Mitchell released figures yesterday indicating that, in comparison to the committee bill, his amendment would give a larger tax cut to the poorest taxpayers -- 72 percent instead of 63 percent -- and a smaller tax cut to those earning more than $200,000 per year -- 2.5 percent instead of 4.7 percent.

Earlier yesterday, the Senate agreed, 51 to 44, not to restore the deduction for charitable contributions for taxpayers who do not itemize deductions. Along with a close vote earlier this week against retaining the deduction for Individual Retirement Accounts for all taxpayers, the failure of the charitable-contribution amendment was seen as a sign senators were continuing to hold out against substantial changes to the Finance Committee's bill.

The chief proponent of the Unocal tax break, Sen. Pete Wilson (R-Calif.), charged that Packwood had freed his troops to vote for the amendment because Wilson had voted in favor of the IRA and charitable-contribution amendments.

"It was retaliation," Wilson said.

A Packwood aide said they had "looked at the close votes. We didn't get any help from the senator from California. Why should the coalition work for him?"

Sen. Alan Cranston (D-Calif.) also spoke in favor of the Unocal tax break and voted for the IRA and the charitable-deduction amendments.

Senate Majority Leader Robert J. Dole (R-Kan.) and Sen. Bill Bradley (D-N.J.), leading proponents of the tax bill, voted with Packwood and others to remove the Unocal transitional rule.

Metzenbaum called the tax benefit the most questionable of the $5.5 billion in transitional rules in the committee's bill. Unlike most of the special rules, this one would create a loophole that is not in the current tax code.

It was needed, Wilson said, to help compensate Unocal for the costs of its defense against last year's takeover attempt by Mesa Petroleum Corp. To fend off Mesa, Unocal borrowed $4.2 billion and used the money to buy back 50 million shares of its stock.

A condition of the debt was that the money be held in a subsidiary engaged in foreign operations, rather than the holding company, so that the lender would have assets to seize if Unocal defaulted.

That action reduced the foreign tax credits Unocal could take against interest earned abroad. The break effectively would let the foreign tax credits be taken as if the funds were held domestically.

The amendment also included a carrot that attracted support from farm-state senators: It would require that the $50 million in tax revenue raised by repealing the Unocal benefit be used to allow farmers to use income-averaging in calculating their taxes.