The Senate Select Committee on Intelligence has decided not to hold hearings on charges by a former Central Intelligence Agency analyst that his secret evaluation of Mexico's political situation was altered by CIA Director William J. Casey.

John Horton, who resigned last May as the National Intelligence Council's chief Latin America analyst, said in a September interview that Casey had pressured him to make the changes and that Casey made them "over my dead body, so to speak."

Other sources familiar with the case said Casey's changes painted Mexico's internal situation as more dangerous than Horton thought justified. They said the agency apparently had planned to use the report when Mexican President Miguel de la Madrid came to Washington in May, in an effort to make him more receptive to U.S. views of growing leftist influence in Central America.

Senate Minority Leader Robert C. Byrd (D-W.Va.) said he was "shocked" by Horton's charges and asked the intelligence committee to investigate and hold hearings on them. In a letter to Byrd, committee Chairman Barry Goldwater (R-Ariz.) refused, saying that insufficient time remained in the congressional session. A committee spokesman said yesterday that the refusal did not mean that the committee had dropped the matter; new information could lead to a probe.

Horton, reached at his home in Maryland, said the committee's decision "is entirely up to them. It doesn't concern me one way or another." He said he had prepared an article on the incident and was awaiting official CIA agreement that it contained no classified information.

Committee officials said that their evaluation of the report found that it did not clearly back any one view of Mexico's situation nor advocate a single course of action.