House-Senate conferees on next year's defense authorization bill agreed yesterday to end a moratorium on production of chemical weapons that has lasted since 1969 and let the Pentagon start assembling new "binary" nerve gas artillery shells in the fall of 1985, congressional sources said.

The agreement, which caught House and Senate opponents of chemical weapons by surprise, came in the second day of the conference on differences between House and Senate versions of a bill authorizing some $200 billion in defense programs for fiscal 1984.

The conferees also agreed to accept a House amendment, drafted originally by Rep. Les Aspin (D-Wis.), tying deployment of the new MX missile that Congress has just approved to simultaneous progress in development of a smaller, less threatening missile nicknamed Midgetman.

The Senate conferees turned down, however, another House amendment, from Rep. Albert Gore Jr. (D-Tenn.), which would have limited fiscal 1984 production to 21 MX missiles, instead of the 27 sought by the administration.

Aspin and Gore were leaders among moderate House Democrats who supported the MX earlier this year in return for administration pledges to take new steps on arms control and work toward replacement of MX with the smaller missile in the future. They offered their amendments to hold the administration to its part of the bargain.

Sources said yesterday that if the Senate did not change its mind on the Gore proposal to reduce fiscal 1984 MX missile production the entire defense conference report could face defeat in the House.

Other sources said even the nerve gas vote was considered "tentative" and could be changed as the conferees went on to other controversial issues in the two separate bills.

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) said yesterday that he hoped the conferees would finish within the next two days so that Congress could take final action on the measure before the August recess begins the end of this week.

Opponents of the chemical weapons decision, however, said last night that they would work to reverse the conferees' decision if the report is brought to the House floor this week.

They are particularly incensed because in June the House specifically turned down by 14 votes an amendment similar to the production proposal the conferees approved yesterday.

The House then went on to approve an amendment that eliminated all production money in the fiscal 1984 for the new artillery shell by an even larger 95-vote margin.

In July, however, nerve gas suporters won in the Senate when Vice President Bush broke a 49-to-49 tie on an amendment to block produciton. The Senate went on to approve a bill which included not only production funds for the binary artillery shell, but also money to begin buying production equipment for a new nerve gas bomb called the Bigeye.

Given the nature of the Senate vote, nerve gas opponents in both chambers said they believed that the House position would prevail in the conference.

However, most of the senior members of the House Armed Services Committee, who were the House conferees, favored resuming production, and yesterday gave in to the Senate position.

They did, however, add an amendment requiring one old chemical shell to be destroyed for each new one built.

A spokesman for Rep. Clement J. Zablocki (D-Wis.), a leading opponent of the measure, said yesterday that the congressman "would fight the conference report when it came to the House floor for a vote."

Proponents of the binary nerve gas weapons have argued that the newer weapons would be safer than the old ones now deployed.

They are made up of two toxic chemicals, but they do not turn into deadly nerve gas until they are mixed together, an action that does not take place until they are fired.

Supporters also have argued that by building new weapons they would encourage the Soviet Union to become more serious about reaching an agreement that would bar all types of chemical weapons.