A federal jury rejected yesterday an Arlington insurance agent's claim that ABC News and House of Representatives staff members investigating insurance sales abuses among the elderly conspired to damage her business during a secret, 1978videotaping session.

The six-member U. S. District Court jury in Alexandria took only 40 minutes to determine the outcome of the two-day trial, which had been billed as a significant test of the limits of immunity for congressional staff aides.

"There's some initial delight," said House Assistant General Counsel Steven Ross of the verdict, "but we're not pleased the case went this far."

Two other lawsuits are pending in Maryland and New York against the network and staff members of the House Select Committee on Aging over their cooperation in an investigation of alleged fraud in the sale of supplementary health insurance to Medicare subscribers three years ago.

House General Counsel Stanley Brand said last night the Alexandria case marked the first time in memory that congressional aides have been ordered to testify in a trial involving a common damage suit despite a claim of absolute congressional immunity. "It has a tremendous chilling effect," he said.

The suit was filed last year by Glenda C. Brown, 32, who answered an elderly woman's request in 1978 to come to her Arlington house to review her insurance coverage. Unknown to Brown, the woman was an employe of the House committee probing insurance fraud.

Brown recommended that the woman, Margaret Dixon, buy three policies that, according to testimony this week, amounted to "overloading," a practice since outlawed by Congress as a result of the committee's investigation.

During the 90-minute interview with Dixon, Brown was secretly taped by an ABC News crew using a two-way mirror. Part of the session was later used in a three-part series on alleged insurance fraud by then-correspondent Margaret Osmer on ABC's World News Tonight.