Fairfax City, accepting a host of concessions from its Northern Virginia neighbors, decided early today to rejoin the Metro transit system in angrily quit nearly three years ago.

While the areawide Metro compact was not exactly torn asunder by Fairfax City's walkout over high subsidy assessments in 1977, the action prompted concern among other Metro partners about the stability of the system and its ever-burgeoning costs. In recent months, several other area jurisdictions have grown restive in the face of ever-higher projections for future Metrobus subsidies and have begun rumbling about setting up their own bus systems (as Fairfax City did) while retaining only Metro rail links.

Fairfax City's return to the regional fold came at an all-night meeting of the city council, which voted 5-to-2 to sign a trial six-month contract for Metrobus service.

Summing up the sentiments of most of the council, member Carl J. Hemmer said: "I don't think there's any alternative if we want to be honest with ourselves and pay a fair share of the Metropolitian transit system."

While agreeing to rejoin Metro, the city will attempt to hold down operating deficits by asking for a cut of 30 percent in the number of Metrobuses that have continued to run through the city without its financial support. City officials maintain that most of the people affected by the cuts could shift to the city's privately contracted bus line -- Fairfax City Express -- which carries about 500 commuters daily to and from downtown Washington. The city started the express line after it left Metro and will continue to operate it.

As part of the agreement to get Fairfax City back into Metro, other Northern Virginia localities will pick up $1.8 million in accumulated bus subsidy charges from 1977 to 1980. The charges were deficits incurred by Metrobuses that continued to run through Fairfax City after the city withdrew from the regional compact.

In other concessions, the city will not have to pay its share of Metro rail subsidies -- $118,000 annually -- for at least two years, and possibly not until the Nutley Road subway station that will serve the city is open in 1985, or later.

What Metro gains beyond the restabilization of the compact is an agreement from the city to start paying its share of bus subsidies effective next Jan. 1. While the amount of the city's share is small compared with those of larger jurisdictions -- about $165,000 annually -- the city's refusal to pay it over the past three years has been a continual sticking point in attempts to forge a spirit of cooperation between the city and its Northern Virginia neighbors.

The city left the Metro compact over both the amount it was asked to pay for bus subsidies and the requirement that if pay its subway subisidy years before rail service was scheduled to reach the city.