The State Department acknowledged yesterday that it considered asking Guyanese authorities last June to police Jonestown more effectively, but then rejected the proposal.

In a 16-page letter to the House International Relations Committee, which is investigating the murder of Rep. Leo Ryan (D-Calif.) and the subsequent suicide/murders of more than 900 members of the Peoples Temple last month, the department said it decided that such a request would be unwise "absent some credible evidence of wrongdoing or unlawful conduct at Jonestown."

The House committee, headed by Rep. Clement J. Zablocki (D-Wis.) released that State Department document without comment, but sources there described it as disappointing, "self-serving and protective."

The U.S. embassy in Georgetown, Guyana, had made the suggestion to Washington last summer, apparently as the result of charges by temple defector Deborah Layton Blakery.

According to Assistant Secretary of State for Congressional Relations Douglas J. Bennett Jr., who submitted the letter Zablocki, Blakey had told embassy Consul Richard McCoy last May much of the information she later made public in an affidavit, including "the mass suicide rehearsal" that took place at Jonestown last year. McCoy evidently thought her complaint serious enough to urge her "to take her information to U.S. law enforcement authorities," according to the State Department.

For their part, embassy officials in Georgetown asked Washington last June for its view "regarding the desirability of a request to the government of Guyana to exercise its jurisdiction in Jonestown more effectively."

Assistant Secretary Bennett sald the State Department rejected the idea on the grounds that "absence some credible evidence of wrongdoing or unlawful conduct of Jonestown, a U.S. government request to investigate the activities of the People Temple might well have raised legal and policy issues related both to concerns for the privacy of U.S. citizens and for freedoms of association and religion."

In the affidavit she signed on June 15 after returning home to San Francisco, Blakey, once a high-ranking aide to Temple leader Jim Jones, said the situation in Jonestown "threatens the lives of United State citizens." She said Jones had become "genuinely irrational" and she recounted repeated mass suicide rehearsals "at least once a week," in addition to a threat by Jones to bring it about in September 1977.

The State Department letter was a reply to a series of questions posed by Zablocki last Nov. 21.