The seizures that sometimes afflict John Denny six times a day began soon after he was struck by a bicycle and suffered a head injury. He was 6. It was 1953.

Today, he illustrates part of America's unfinished agenda of justice. His mother's plea to Maryland's legislature is for a small reform suited to today, a cost-effective demonstration of a state government's competence and compassion.

The day I visited Mary Denny in her small apartment in Silver Spring, John had just had a seizure, fallen and smashed a chair. Recently he went five days without a seizure, but once he had 37 in 24 hours. When seizures occur, she must protect his head so he does not have to go to the emergency ward yet again. Then she must get him to bed. It's okay, she says, "he doesn't weigh much more than I do." She is 75, a widow, suffering angina. She is 5 feet tall.

She is not a complainer: "It's difficult at times, of course, but it's difficult for him, too." Neither is she clamorous. "I'd like him to be involved in something." She means a group home, a daytime activity, something, anything.

Recently, she journeyed to Annapolis, to the legslature, an arena of high- pitched complaining and muscular demanding. She testified concerning a bill that would do something for John, who has received nothing in the way of community services for 14 years, since he turned 21. John's affliction is abnormally severe. The bill would do more for the many mentally handicapped persons who are shut away from the community even though they have no incapacitating physical complications comparable to John's and could master work skills and public transportation.

The bill would move Maryland away from the unnecessary consigning of handicapped persons to institutions. It would commit the state to emphasis on community-based care in smaller, more open units, such as group homes.

Current policy is especially unjust to persons like Mary Denny. Her special sense of parental responsibility kept her from making John a burden on the state by institutionalizing him. So today she gets no assistance because available funds are drained away by big institutions.

Gov. Harry Hughes is setting a fiscally and morally correct course by closing one large institution, but there are long waiting lists of adults in need of community programs. Nearly 1,800 are in institutions although they have been evaluated as suited to less restrictive environments.

Group homes are the heart of the new approach, and they encounter the NIMBY (Not In My Back Yard) syndrome. Money is the most serious problem. But although care in the community costs more than neglect, it costs less than institutional care. And savings are compounded when, with the help of public- spirited corporations, the mentally handicapped, most of whom are less handicapped than John Denny, find fulfilling employment and become taxpayers. The Marriott Corp., headquartered in Maryland, is a splendid example: 4 percent of its employees in this country are physically or mentally handicapped.

The Reagan administration has wisely authorized states to apply for Medicaid reimbursement for a range of services provided in the community rather than just for costs incurred in the usual--and usually dismaying--large-scale institutions. Furthermore, in an act of responsibility that should shame other interest groups, the Maryland Association for Retarded Citizens has endorsed revenue- raising measures more than sufficient to pay the costs of the transition to community-based programs.

One measure would recover some corporate tax revenues that Maryland would lose because its tax code is coupled to certain federal tax categories that have been reduced. With such a measure Maryland legislators can assert their independence from federal revenue actions.

The bill Mary Denny needs may be a prototype of revivified state politics. By such measures, state goverments can demonstrate that Washington has no monopoly on imagination, and that states will provide themselves with revenue foundations commensurate with the states' minimal duties.

When Maryland's legislators vote, they should picture Mary Denny, her placid face framed by white hair, her tiny hands clasped in her lap. She is a marvel of courage who has been more self-reliant, longer, than any citizen should have to be. She says, "There's no rush, but John might outlive me." To her credit, and disadvantage, she has not mastered what today's most successful petitioners have --an angry, self-pitying tone of voice.