WITH MARYLAND Gov.-elect Harry R. Hughes still assembling his cabinet and with the 1979 General Assembly opening for business a week from next Wednesday, it's still not clear exactly how the executive-legislative relationship will work in Annapolis this year -- or what the two branches of government expect to accomplish. But we do know already that one of the most important missions for the delegations from Montgomery and Prince George's counties will be to secure state assistance for the Metro system. That is why local leaders from the two counties paid a joint visit on the governor-elect this week. Though Mr. Hughes's press aide said "they didn't really ask for anything," we trust that their message was understood by the governor-to-be, whose expertise just happens to include six impressive years as secretary of Maryland's Department of Transportation.

Similarly, we hope that the effort of Montgomery and Prince George's legislators this year will not be read by their colleagues as purely a local issue, or as another round in the sectional competition between Baltimore and the Washington suburbs for money from Annapolis. Just as the various jurisdictions of Greater Washington have come to recognize the mutuality of their transportation needs, Maryland's lawmakers should see the importance of a good transit system throughout the entire Baltimore-Washington region as a sound investment for the state. For that matter, the state already is paying the entire local share for Baltimore's subway and the local share of Metro's capital costs; the greater problem for Metro is its operating costs.

The legislators from the two counties have been talking about earmarking 1 percent of Maryland's 5 percent general sales tax for the state's transportation fund, which could be spent in many different ways around the state -- transit, roads, railroads, etc. This would not involve an increase in the tax rate. It is a reasonable proposition that Gov.-elect Hughes and all responsible representatives in Annapolis could and should support.