Ada County prosecutor Jim C. Harris, whose office executed a search warrant at a television station here last weekend, said today that a reporter for KBCI-TV lost his first Amendment rights when he agreed to listen to prisoner demands at the riot-torn Idaho Penitentiary.

"He was an agent for rioting inmates a conduit of information between inmates and the penitentiary," said Harris. "The station was not acting as a news-gathering source, but a news-generating source, and as such did not have First Amendment rights."

Acting on a search warrant, Harris criminal investigator and five sheriff's deputies went to the station's newsroom saturday. They opened desks, combed the files and took a tape-editing machine and two video cassettes filmed Wednesday during an 18-hour incident at the penitentiary when the inmates holding two guards hostage demanded to air their grievances on television.

Station officials charged that the search violated constitutional protections afforded to the press and said the search compromised some of their confidential sources.

Only a small portion of the film of the prison incidents has been aired. The station's policy is to resist giving prosecutors video tape that has not been televised.

Harris said the film he obtained is duplicate footage and unsatisfactory. He said his staff is negotiating with KBCI attorneys in an attempt to obtain the original footage.

"The whole situation is being studied, said Carl P. Burke, an attorney for KBCI. "It would be a mistake to speculate as to what we would do."