RESIDENTS of Maryland's Fifth District lost the services of an extraordinary representative in Congress when Gladys Spellman suffered heart arrest during a campaign stop two years ago. But in a special election that sent Democrat Steny Hoyer to Congress in June of last year, the district gained a figure whose understanding of state and national legislative processes quickly won him a position of influence. He has worked effectively in the give- and-take that can bring dividends to his district.

His Republican opponent is William P. Guthrie, a 29-year-old doctoral candidate, who barely campaigned in the primary and has made limited appearances since. He has advocated more money for defense, with cuts in "so-called 'social welfare' and 'domestic reform' programs." This looks to be no contest, with good reason. Mr. Hoyer has earned a chance at his first full term.

The Eighth District again fields articulate and informed candidates from both parties. The distinction this year is that Democrat Michael Barnes has emerged as this region's most able member of the House on national and local issues alike. His Republican opponent, Elizabeth Spencer, differs little on substantive economic and social matters, though she does criticize him for voting against the Reagan 1981 tax-cut bill. Mrs. Spencer has contributed thoughtfully to joint appearances with Mr. Barnes, but she has yet to seize upon any dramatic differences that might diminish his standing.

In the Fourth District, Republican Marjorie S. Holt is running for her sixth term, still a staunch supporter of solid spending for the military but for little else. Her Democratic opponent, former delegate Patricia O'Brien Aiken of Annapolis, would put more money into domestic social programs and public works projects. Her more than 20 years in local politics have won her a reputation as a knowledgeable candidate, perhaps the strongest yet to challenge Mrs. Holt.

In the Sixth District, Democratic Rep. Beverly Byron has demonstrated an interest in a wide variety of issues from finances to energy. As one of only two northern Democrats to support the Reagan budget in the House in 1981, Mrs. Byron continues a party-crossing habit that has had great appeal to voters in her district. Her Republican opponent, Roscoe G. Bartlett Jr., describes himself as a farmer-businessman-teacher. Over the years, he emphasized his opposition to gun control and has called for unspecified but deep cuts in every function of government except defense. Mrs. Byron is clearly the better choice.