THE NATIONAL TRUST for Historic Preservation has just made an impressive investment in the future of Washington's past. It bought the McCormick Apartments, a fine 50-year-old building at 1785 Massachusetts Avenue NW, from the Brookings Institution for $1.34 million. The stately beaux arts structure was once the ultimate in luxurious apartment houses, with a long list of celebrated occupants that included Andrew Mellon, Robert Woods Bliss, Perle Mesta and Sumner Welles. While using the five-story complex for offices, as it has been used since 1941, the Trust intends to remodel extensively, at a cost of $1.6 million, and restore much of the splendor of the building's early days.

In moving uptown from Decatur House, the National Trust is doing more than shifting from a building associated with the city's - and the country's earliest years to one rich in the cultural and social history of the early twentieth century. The Trust is also moving the core of its operations from Lafayette Square, whose preservation seems assured, to a part, of central Washington whose future is very much unsettled now. Many of the splendid older buildings along that stretch of Massachusetts Avenue and nearby thoroughfares such as 16th Street have already disappeared. Those that remain are under constant pressure from age, disrepair and redevelopment. Thus the Trust has chosen both an ideal time and place to show other institutions how a historic downtown property can be rejuvenated and preserved.

The Trust's example ought to be studied carefully by federal agencies, other non-profit groups and foreign governments that desire more consular space. Some major law firms and other private establishments might also be attracted by the possibilities for adaptive use of such distinctive space - especially since the new federal tax alw includes important incentives for preservation of historic properties. Before those provisions can be fully used downtown, however, local policies will have to be modified in two respects. The District's own tax system will have to be revised to encourage renovation and maintenance of buildings that are worth keeping. And there will have to be some relaxation of the zoning policies that now restrict large parts of major corridors to governmental and non-profit office use. Where profit-making firms do want to follow the National Trust's example, the city should certainly not discourage them.