If suburban Maryland voters sent any message to Annapolis in last week's election, it was that incumbency and seniority really do count for something in the generally low-visibility campaigns for the Maryland General Assembly.

In Montgomery County, where tradition holds that each four years reform-minded challengers knock off old-line incumbents, voters this year decided to stick with known quantities. As a result, Montgomery will return five of its six state senators and 15 of its 18 House delegates when the General Assembly session opens in January.

In Prince George's, the self-proclaimed "independent Democrats" in the September primary election overpowered a number of County Council candidates affiliated with the old Democratic organization. But neither the "independents" nor any Republicans made inroads in the state legislature, and as a result, six of a possible seven incumbent state senators will return to Annapolis. Four ran unopposed in the general election.

They will be joined by 19 incumbent delegates out of a possible 21. And in all but one case in Prince George's, the voters even turned back attempts by some senators to replace incumbent delegates on their slate with newcomers of the senators' choosing.

Seniority is not unimportant since the next legislature is to take up key fights over state funding to the counties, including police aid, transportation aid and education allotments. Both Montgomery and Prince George's will be returning delegations with seniority and some key committee chairmanships.

In Montgomery, Sen. Laurence Levitan chairs the Budget and Taxation Committee; Del. Helen Koss chairs the Constitutional and Administrative Law Committee; Del. Joseph E. Owens heads the Judiciary Committee and Donald B. Robertson is the majority leader. Prince George's Del. Gerard F. Devlin is vice chairman of the House Ways and Means Committee.

If an agreement between Prince George's and Baltimore County senators holds up, Prince George's is likely to gain two leadership positions in the Senate. In exchange for Prince George's support of Sen. Melvin A. (Mickey) Steinberg (D-Baltimore) in his bid for the Senate presidency, Sen. Thomas V. (Mike) Miller, president of the Prince George's delegation, has been promised the posts of majority leader and chairman of the Judicial Proceedings Committee.

Sen. Tommie Broadwater Jr. has also been promised the chairmanship of a budget subcommittee dealing with law enforcement and transportation. In the House, however, the Prince George's delegation will miss the services of Del. Robert S. Redding, the chairman of the county's delegation, who was defeated in September after a last-minute run at the 22nd District Senate seat held by Thomas O. Reilly. Delegation members considered Redding a fair leader who kept internal disputes to a minimum.

The big losers this year were reform-minded liberals, like Montgomery's delegate candidate Gilbert Genn, as well as some publicity-minded gadflies, like Montgomery's Robin Ficker and Prince George's Senate candidate, Del. Charles Blumenthal.

Blumenthal, for years a thorn in the side of fellow legislators with his attention-getting stunts and rhetoric, was bested by incumbent Sen. Frank Komenda, who had help from fellow senators. Ficker came in fifth in a field of six in his effort to be reelected to a seat in District 15.

Genn, on the other hand, tried to run an "issues oriented" campaign and refused to become part of the Democratic ticket in Montgomery. Democratic incumbents in Montgomery were criticized last spring when they first took a page from the book of Prince George's politics and began floating the idea of an incumbents' slate. Candidates like Genn spent their entire campaigns accusing the incumbents of "machine politics" and promising to bring in new blood.

But in the end the only three new faces in the Montgomery delegation, and the three in Prince George's, will be candidates who won the Democratic Party's approval. Delegates-elect Gene Counihan in the 15th District, Michael Gordon in the 17th, and Diane Kirchenbauer in the 20th all benefited from running on the party slates in their districts.

In the 16th District, Genn won the primary by beating a weaker, party-backed candidate, but had to run a separate campaign in the general election with little help from the two incumbent Democrats. It was late in the campaign before Genn and the two incumbents even had joint posters printed, and Genn was forced to rely on his own cadre of workers drawn largely from the animal humane movement.

In Prince George's 23rd District, Leo Green will replace Edward Conroy, a senator who died of cancer earlier in the year. Green narrowly lost the seat to Conroy four years ago after a bitter campaign, but was embraced by the rest of the "Democratic Alliance" this year.

In the 25th District, newcomer Albert Wynn will be joining incumbents Dennis C. Donaldson and Lorraine M. Sheehan. He was selected to join a slate headed by incumbent Sen. B.W. (Mike) Donovan. In the 27th, Marian L. Patterson was elected after filling a vacancy on the Komenda-led ticket created by Blumenthal's losing bid for the Senate.

Members of the Prince George's delegation must also decide who will replace Redding as chairman of their delegation. Candidates for the job include Dels. Lorraine Sheehan, Charles (Buzz) Ryan and William McCaffrey.