Montgomery County Democratic Del. Helen L. Koss, a 16-year veteran of the General Assembly and chairwoman of the House Constitutional and Administrative Law committee, has announced her retirement from public office.

Koss, 64, who gained a reputation as a champion of state ethics and procurement legislation during her tenure, said she decided against seeking another term when her husband, Howard, decided to retire from his contracting supply business.

Koss, who is a past president of the Maryland League of Women Voters, said that she was proudest of her efforts over the years to pass the Maryland Equal Rights Amendment and of the success of legislation to aid displaced homemakers and provide equal credit opportunity for women.

She has headed her committee since 1979, when she helped House Speaker Benjamin L. Cardin shepherd a broadened ethics bill through the legislature. This year the committee helped expand the state's public accommodations law and approved the bill that will make Maryland part of 1988's southern regional presidential primary.

Ethics questions, she said, have come full circle from the "heightened sensitivity" that followed the years after the state's political corruption trials. Now, "That sensitivity has sort of receded," she said. Kasemeyer Runs for Senate

Del. Edward J. Kasemeyer (D-Howard, Montgomery) says he will run for the seat being vacated by longtime state Sen. James Clark.

Kasemeyer, 50, had planned to run for a second term as a state delegate, but he changed his plans when Clark decided not to seek another term.

Clark retired last month after 28 years in the legislature, 24 of them as a senator.

Kasemeyer joins several Democrats seeking the party's nomination, including Frank Blunda of Montgomery County; Hugh Burgess, who served four terms in the House before being defeated by Kasemeyer, and Del. Robert Kittleman (R-Howard, Montgomery).

The winner of the Democratic primary is expected to face Republican newcomer Chris McCabe of Columbia. Perry Seeks Senate Seat

Prince George's Democrat Jerry E. Perry, who was appointed to the House of Delegates in 1983, has announced her candidacy for the state Senate from the 25th district, which includes District Heights and Morningside.

Perry, 41, joins Del. Albert Wynn in the contest against incumbent B. W. Mike Donovan, who has not yet said whether he will run again for the seat he has held since 1979.

"I do not seek to oppose any individual or any group of people," Perry said during a recent fund-raiser. "I seek to propose sound principles of leadership for a growing Prince George's County." Booker Enters House Race

Edith Booker, 39, a state personnel administrator with a doctorate in botany, announced last week that she will seek election to a 23rd District House of Delegates seat in Prince George's County.

Booker, a Seabrook resident who was one of several candidates for the vacancy created by Del. Gerard Devlin's appointment to the District Court bench, said that she hopes to broaden the district's representation in Annapolis beyond the Bowie area, where all of its elected state officials now live. Union Endorses Bobo

The Howard County Education Association is endorsing Democrat Elizabeth Bobo for County Executive. She sits on the County Council. Father, Son File

Xavier Aragona, a Prince George's County attorney who is running for the House of Delegates in the 27th District, filed as a candidate for office on the same day last week as his son, Christopher, 23, who is running for the Democratic State Central Committee of Maryland. Both live in Fort Washington.

Xavier Aragona, 51, served in the House of Delegates from 1966 to 1970 and ran unsuccessfully for U.S. Senate in 1974. Shelby to Run Again

Paul R. Shelby, chairman of the Prince George's County Board of Education and a member of the board since 1982, has announced his candidacy for reelection from the 5th District.