THE GENERAL PURPOSE of the Foreign Missions Act of 1982, now working its way through Congress, is an important one: to address a growing imbalance between the treatment of U.S. government missions abroad and that accorded foreign missions in this country. The measure would provide a better system by which the United States could confer, deny and limit the various privileges, benefits and immunities enjoyed by foreign governments in this country.

A new Office of Foreign Missions would be established in the State Department, which could strengthen the hand of State in dealing with other nations on the placement of embassies and chanceries and related reciprocity questions. That's all well and good, but where a number of senators get off--and where the District's limited home rule authority would be severely and unneccessarily curtailed--is in two sections of the measure calling for crude federal preemptions of local authority.

The first, Section 206, would create a Foreign Missions Commission of D.C., federally weighted in its membership and empowered to decide the location and use of land for chanceries (business offices of foreign governments) in the District. Also, Section 207 of the measure could be construed as permitting an open- ended State Department preemption of decision-making processes in 213 cities that have foreign chanceries in them--a source of understandable concern on the part of such groups as the U.S. Conference of Mayors, the National League of Cities and others.

Sen. Mathias, who heads the subcommittee handling D.C. affairs, and Sen. Eagleton, ranking minority member, led a unanimous subcommittee vote for amendments to avoid any unneccessary interference with local authority, but the full committee-- in a divided vote, with five abstentions--disagreed, leaving any amendment effort for the coming floor debate.

Both senators have pointed out that the current D.C. home rule act approved by Congress in 1973 already contains protections of the federal interest through representation on the D.C. Zoning Commission as well as the D.C. Board of Zoning Adjustment. They also note that these two groups, along with the National Capital Planning Commission, which is dominated by federal representatives, have always coordinated with the State Department on chancery locations in the past anyway; and that adding another commission would be wasteful and pointless, while damaging local participation.

"What case," Sen. Mathias asked his colleagues the other day, "exists to overturn a process which clearly is working?" None--which is why senators who give this otherwise significant measure their serious thought will vote for the Mathias amendments to correct dangerous and unneccessary federal preemptions of local authority.