Defense Secretary Caspar W. Weinberger leaves for Portugal today for a NATO meeting where he will be under heavy pressure to demonstrate U.S. willingness to be flexible in negotiating reductions in medium-range missiles deployed in Europe.

As he discusses that and other issues with counterparts in a Nuclear Planning Group meeting in the Villamoura resort near Faro, Portugal, administration officials in Washington will be drafting alternatives to the president's "zero option" proposal.

Under the zero option, the United States would forgo the planned deployment in Europe of 572 cruise and Pershing II missiles if the Soviets retired their 351 SS20 missiles threatening the continent. Joseph Luns, NATO secretary general, labels the zero option "not attainable" in an interview to be broadcast over Belgian television today.

Until Reagan decides whether he wants to put some kind of half-way proposal on the negotiating table, or stick to the zero option, Weinberger will be limited in the amount of assurance he can give fellow NATO defense ministers. Many of them fear massive demonstrations will erupt in their nations unless Reagan changes what many European leaders regard as an all-or-nothing-at-all approach to European missile deployment.

Weinberger's position is that the United States has put forward a bona fide missile reduction proposal and it is now up to the Soviet Union to respond formally, including offering alternatives, rather than just say "nyet" to the zero option.

He believes backing away from the zero option at this point would be a mistake; that the Soviets will become serious about reductions only after U.S. deployment of the cruise and Pershing missiles is under way.

Army Gen. Bernard W. Rogers, commander of U.S. forces in Europe, took the same position before a Senate Armed Services subcommittee last week and warned that if cruise missiles are not deployed on schedule, starting in December, it would be "disastrous for the alliance because we do have a gap in our spectrum of deterrents which we have to fill."

Whether Weinberger or those in the administration who favor alternatives to the zero option will win this latest struggle for the president's mind will be the big question hanging over the NATO meeting.

Other issues the defense ministers are expected to discuss in the corridors, if not around the conference table, include the utility of NATO's existing arsenal of tactical nuclear missiles and whether the United States is being fair in regard to how many weapons it buys from NATO partners compared with how many it sells to them.

The Netherlands is particularly angry over what it considers the failure of the United States to travel a two-way street in NATO procurement and is threatening to stop buying American weapons if changes are not made.

From Portugal, Weinberger is scheduled to go to Madrid on Wednesday to help reinforce the stand of newly elected Socialist Prime Minister Felipe Gonzalez, who as opposition leader criticized the Spanish government's decision last summer to enter the alliance.

Gonzalez said last week he will not carry out any time soon the Socialist promise to hold a referendum on whether Spain should remain in NATO. This might weaken the alliance at a time of high East-West tensions, he said.

Weinberger is scheduled to fly from Madrid to Washington on Friday.