THE THREE-PART SERIES on mental retardation by staff writer Robert Meyers, which appeared in The post this week, was a sensitive tribute to Mr. Meyers' parents and his retarded brother, Roger. Mr. Meyers described the frustrations and anxieties of his parents when faced with the uncertainties of how best to raise their retarded son. He pinpointed the moments of pride when they began to realize that Roger Meyers, largely as a consequence of his own stubborn will, was breaking through the presumed limits of his condition. But the series offered more than a stirring reminiscence. It also provided a valuable guide to how far we have come in the past 15 years in out ability to help the mentally retarded - and how far we have yet to go.

The bleak scenes of life in crowded mental institutions are vividly etched in many of our minds. Books, films and reports have exposed the wretched conditions of what were once called "insane asylums." Not too long ago, many in the country shared the belief that mentally retarded persons should be locked away - period. But the hard work of physicians and parents has changed much of that. Now attention is centered on giving retarded persons the kind of help and guidance that allow them to overcome as many of their troubles as possible and live ever more nearly normal lives. There has also been a growing realization of the importance of noninstitutional care - residences in which retarded persons can enjoy many of the freedoms and responsibilities available to others. Job-training programs, family-counseling services and diagnostic-testing procedures have all been increased, helping both retarded persons and their families to understand and cope with retardation and its social and emotional effects.

All this came into play in the efforts to rescue Roger Meyers from a dull fate that - interestingly and movingly - he himself resisted powerfully. But the more reasonable approach, from which he benefited, is far from widespread. Most mental-health facilities in the country are still below standard. As an example, in a report that was released this week Maryland's 11 mental institutions are said to provide "deplorable and sometimes inhumane" conditions for the 7,000 patients how living in them. The condition of some federally supported facilities, such as St. Elizabeths, is much the same. There is not much money for improving these institutions or expanding the number of foster homes and halfway houses. The availability of professional counseling is limited and, in some cases, so expensive as to be prohibitive. And, for all the new enlightenment, prejudice remains strong. Nevertheless, the Meyers family found a way to overcome the obstacles. Their story, while perhaps exceptional in some respects, serves also as a good general example. It showed what can be accomplished by persistence, patience and the will to succeed.