Senate sergeant-at-arms Henry K. Giugni, the second-ranking officer of the Senate, announced yesterday that he will retire at the end of the year.

Giugni said in a statement he would join the Washington lobbying firm of Cassidy and Associates Inc. as vice president for corporate development.

Giugni's four years as head of the largest office in the Senate were marked by major changes, including the multimillion dollar purchases of new computer and telecommunciations equipment for members and the expansion and modernization of their home state offices.

In his statement, Giugni said he was "particularly proud" that he had helped make the Capitol accessible to the disabled by expanding the Special Services Office, which provides tours for the blind and publishes Braille versions of Senate documents.

Giugni was elected sergeant-at-arms in January 1987 after serving 23 years as an aide to Sen. Daniel K. Inouye (D-Hawaii). Prior to that he had been a member of the Honolulu police force.

The Senate sergeant-at-arms presides over a $120 million budget that pays for the Senate computer center, the radio and television recording studio, post office, printing facility, telecommunications and office equipment, and the Senate contingent of U.S. Capitol Police.

By tradition, the Senate majority leader, currently George J. Mitchell (D-Maine), names the sergeant-at-arms. A spokesman for Mitchell said yesterday was not ready to name a replacement for Giugni.

Giugni was selected by then-Majority Leader Robert C. Byrd (D-W.Va.), reportedly as part of a deal that kept Inouye from challenging Byrd that year for the leadership. When Mitchell upset Inouye for the leadership post for the 101st Congress in 1989, he agreed to keep Giugni.

As Senate sergeant-at-arms, Giugni also served as one of three members of the U.S. Capitol Police Board, which provides policy direction to the 1,340-officer Capitol Police force.

During his tenure, Giugni was involved in several disputes with House sergeant-at-arms Jack Russ, including differences over how to unify the separate House and Senate Capitol Police payrolls. Unlike Russ, whose policies for police on the House payroll were monitored by the House Administration Committee, Giugni had a relatively free hand in the Senate and introduced the use of civilian guards to handle activities that he thought did not require unformed officers.

This year, the Congress voted to direct the House to introduce civilian guards as replacement for police officers on some jobs in its fiscal 1991 legislative appropriations bill.

Giugni ran into criticism last year for using official funds for repeated trips to his home state of Hawaii. He returned more than $2,000 to the U.S. Treasury after news accounts about his trips.

He also was described as favoring some Capitol Police officers, promoting them to the rank of special technician in a practice that was outside the normal competitive promotion system.

Complaints of alleged racial and job discrimination by some members of the Capitol Police force over the past year led to legislation that eliminated such promotions and introduced an ombudsman to handle future discrimination complaints.

Russ, who also was criticized this year for using off-duty Capitol Police officers and equipment for a prank, said yesterday he had been told by his boss, House Speaker Thomas S. Foley (D-Wash.), that he will be nominated for another two-year term as House sergeant-at-arms.

Capitol Police Chief Frank A. Kerrigan, another top member of the Hill staff bureaucracy, has been rumored to be contemplating retirement in June when he completes 25 years of service, a congressional source said yesterday. A spokesman for Kerrigan was unavailable for comment yesterday. In the past, the chief has declined to discuss his personal plans.