The Agriculture Department and the Central Intelligence Agency were overestimating the size of this year's Soviet grain harvest by 10 per cent as late as Tuesday or early Wednesday of this week, according to authoritative sources.

The announcement by Soviet leader Leonid I. Brezhnev early Wednesday that the 1977 harvest would be 194 million metric tons "caught both agencies completely off base," one of the sources said yesterday.

The sources indicated that both the CIA and the Agriculture Department were still estimating the Soviet crop at 215 million metric tons when the announcement was made by Brezhnev. A metric ton is 2,205 pounds.

Agriculture Secretary Bob Bergland said at a press conference this week that one reason for the sudden discrepancy between U.S. estimates and actual output may have been the bad weather which impeded the final stage of the harvest. Bergland indicated that the conditions for harvesting the crops were unusually poor and could have caused substantial losses.

Although the development surprised both agencies, there has been talk in the private grain trade for weeks that the Soviet crop was smaller than the department was estimating. The trade also has been estimating that total grain sales to the Soviets would be larger than the department has acknowledged publicly.

The department has kept regular tabs on Soviet harvest developments, including the use of CIA and other information, since surprise large wheat and corn purchases by the Soviet Union in 1972 helped send world grain prices soaring.

A special department task force, which has the responsibility of estimating Soviet grain production periodically during each crop season, clung to the 215 million tons previously estimated because of "too many uncertainties" involved, one source said.

However, the task force was not unanimous. There was some feeling that the estimate of 215 million tons should be reduced five million to [WORD ILLEGIBLE] million tons.

After Brezhnev's disclosure, Bergland held a news conference late Wednesday at which the department announced its new estimate of 194 million tons, the same as Moscow's figure.

Bergland also said he would not be surprised if the Soviets bought 15 million tons of U.S. wheat and corn in the 1977-78 year which began Oct. under a long-term trade amgreement committing Russia to buy a minimum of six million tons annually.

The 15 million tons is the amount the United States a month ago told the Soviets they could buy this season without further consultation. They bought the minimum of six million tons in 1976-77, the first year of the agreement.