For the first time since Americans conceded in the 1800s that Washington was, indeed, the Nation's capital and federal agencies became the major source of employment, the United States government has been replaced as the area's top provider of jobs.

As of the end of 1978, the federal government was edged out among employment sectors in metropolitan Washington by jobs in the service industry -- a broad category of people, many of whom spend their time trying to figure out what the government itself is up to.

In January, according to the D.C. Department of Labor, metropolitan service industry employment totaled 360,300 compared with 359,700 persons working for the federal government. A year earlier, there were 353,100 federal workers and 344,100 in the services industries.

That represents a one-year increase of some 16,000 jobs in services while the federal payroll increased only 6,600 with most new jobs in both sectors located in the suburbs of Washington.

For the District alone, the federal government remains far ahead with a January payroll of 227,600 workers compared with 225,800 a year earlirer. E.C. services employment reose more rapidly, however, to 159,600 from 152,800.

Combined with separate statistics on personal income, the new jobs figures show that services are the fastest-growing sector of Washington's economy. The D.C. government payroll actually declined from 48,400 at the beginning of 1978 to 47,000 in January.

Included in the services sector are such professionals as doctors, architects, accountants and lawyers -- many of them watching after governement workers or monitoring government activities for their clients.

Other services jobs are in the tourism business (hotels, motels, tour guides) or individual services, such as barbers.

A major factor in the sharp growth recently has been the trade association business, which now employs close to 50,000 Washington area residents. The D.C. region supplanted New York City as the trade association capital earlier this decade, and this business is growing month by month as business and professional groups have concluded they most be able to communicate quickly with policymakers, legislators and government workers who implement the decisions that are made.

One month ago, the National Restaurant Association announced that it will move here from Chicago. With a budget of $6.5 million a year, the NRA is one of the nation's 20 arges trade groups.

This past week, the National Association of Certified Mortgage Bankers -- representing leading technical, financial and managerial experts in mortgage banking -- moved here from Nashville. In Fairfax County alone, the number of trade and professional groups has increased from two to 125 in the past decade.

When the federal government was established here at the turn of the 19th century, the area consisted of two small, merchantile and port communities -- Georgetwon and Alexandria. By the mid-1820s, the federal payroll was up to 300 out of an area population of 33,000.

Various historic developments -- the Jacksonian era, Civil War, World Wars and Depression in this contury -- all contributed to establishing to federal payroll as dominant.