AS MONTGOMERY COUNTY'S population becomes more diverse, the demands on its government are changing. That's the central point for voters to keep in mind as they go to the polls Tuesday to choose the eight people -- the county executive and the county council -- who will run it for the next four years.

Economic development is proceeding successfully, but there's a need for job training as well as for jobs. Finding the money for urban transit is going to be central to the county's finances, and the importance of good service is increased by the rising proportion of elderly people in the county. Who is going to preside over the competition between the young and the old for public resources -- or the competition between the upper half of the county, where new schools still have to be built, and the lower half, where they are being closed? The county now has significant black, Hispanic and Asian minorities. Is the current quarrel over school integration going to be carried over into housing policy?

As county executive, Charles W. Gilchrist has been addressing these kinds of questions with decency and a notable breadth of concern. He is unquestionably the better choice for the job. The challenger, Joseph C. McGrath, has run a vigorous campaign but one that doesn't reflect the kind of deep familiarity with the county and its public life that the executive will need in the term ahead.

The county council has turned into nearly a full- time legislature. For the two candidates to be elected at large, the strongest combination, we think, is one Democrat, Neal Potter, and one Republican, Leonard Robinson. Mr. Potter, a thoughtful and independent man, brings long experience to the job. Mr. Robinson, in contrast, has a habit of asking the right questions for a newcomer -- whether, for example, that gigantic incinerator planned for the center of the county is being carried forward by anything more than inertia.

The decision in the First District is genuinely difficult, for here again the Republicans have put up an impressive candidate in Alvin Arnett. But Scott Fosler's careful and intelligent work over the past term entitles him to reelection.

In the Second District, the Democratic incumbent, Esther Gelman, is running against a Republican, Michael C. Helmentoler, with little previous record of interest in county affairs. In the Third, the stronger candidate is William E. Hanna Jr., for four terms the mayor of Rockville and an experienced and responsive politician. In the Fourth District, neither candidate has a notable record and it won't make much difference who wins. In the Fifth, the incumbent, Rose Crenca, has made herself a frequent defender of neighborhood interests. That often leads her into a narrowly parochial perspective. But in a very large county, with its highly bureaucratic and professional system of land planning and zoning, there's a case for keeping one person like Mrs. Crenca on the county council.