Just as the Senate begins considering legislation to overhaul the tax code, the Treasury Department is losing its two top lobbyists and two high-level tax lawyers.

The assistant secretary for legislative affairs, Bruce E. Thompson Jr., will head the federal-relations office of Merrill Lynch & Co. Deputy assistant secretary Mimi A. Feller has already become vice president of government relations for Gannett Co. in Rosslyn.

The Treasury also recently lost its chief tax expert, assistant secretary for tax policy Ronald A. Pearlman, and Mikel Rollyson, head of the tax-legislation office.

Thompson said Treasury Secretary James A. Baker III is "talking to people right now" to find his replacement, and none too soon: the Senate Finance Committee begins hearings Jan. 29 on overhauling the tax code.

Tax revision, passed by the House late last year, is Baker's chief domestic initiative. He and his deputy, Richard G. Darman, did much of the negotiating with reluctant House Republicans. Both are experienced on Capitol Hill. "It's not like they need someone telling them what to do," Feller said.

But Senate consideration is expected to be lengthy, and senators may be even more skittish than House members about curtailing deductions and reducing rates.

Although Finance Chairman Bob Packwood (R-Ore.) says he wants to push a tax-overhaul bill through the Senate, he has not shown the enthusiasm demonstrated by House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.). Nor does he have the close working relationship with Baker that Rostenkowski has.

In a statement, Baker praised Thompson's "valuable contribution" to the department. Thompson will work for the firm once headed by his ex-boss, former treasury secretary Donald T. Regan, now White House chief of staff.