The Senate, struggling to agree on next year's budget, yesterday took up an unfinished deficit-reduction legislation from last year and found itself back in a deadlock with the House.

It approved a measure that would cut spending and raise revenues by $18 billion over three years, after modifying it to satisfy the Reagan administration. But the Senate bill conflicts with a version of the same measure passed earlier by the House.

The deadlock carries a price tag. The measure includes a permanent extension of the 16-cents-a-pack tax on cigarettes, which drops back to 8 cents a pack today under a temporary extension approved earlier as Congress haggled over the budget bill. Until Congress reimposes the 16-cent levy, the tax will remain at 8 cents, although the budget bill is designed to apply the higher rate retroactively.

The House next week will face the choice of insisting on its version or ending the impasse by agreeing to the Senate-passed measure. But the House has voiced sharp disagreement with some Senate provisions and the White House has threatened a veto of the House version.

The bill would modify many federal programs to cut costs and includes provisions favorable to tobacco and oil-producing states, which have become the principal forces working for its passage. It would bail out the tobacco price-support program, set states' share of receipts from outer continental shelf (OCS) oil and gas exploration and limit Medicare reimbursements, among other provisions. The House bill would extend welfare benefits to two-parent families; the Senate bill would not.

In another major difference, the House bill would give coastal states veto authority over federal decisions to allow OCS drilling. A move by Californians to include the same provision in the Senate version was rejected, 53 to 35. To help win administration approval, the Senate also dropped a House-approved "Buy America" provision to require OCS drilling structures to be at least half U.S.-made.

The three-year savings of less than $20 billion in both bills is considerably less than the nearly $75 billion in savings envisioned when the measures started their congressional journey last summer. Time eroded some of the savings, and Senate Budget Committee Chairman Pete V. Domenici (R-N.M.) said other savings were included in legislation already passed by Congress, such as the 1985 farm bill.

Congress may not be able to "take much pride" in the speed with which it dealt with the measure, Sen. Lawton Chiles (D-Fla.), said during yesterday's debate. But "at least [it] showed a new level of persistence," Chiles added.