Fairfax County, in a controversial display of suburban self-reliance, this weekend will become the third major locality in the Washington area to break away from the regional Metrobus system and start its own bus service.

The county's chief motive is clear-cut: money. Officials have predicted savings of $735,000 in the first year and possibly a lot more in the long run. "There's money to be saved," said transit operations chief Andrew J. Szakos. "So far it looks good."

But the long-debated opening of the county's system, called the Fairfax Connector, is sending tremors through other local governments and transit agencies, worried about the balkanization of Washington's transit network and eventual dismantling of the 12-year-old Metrobus system, which was meant to hold the region together.

With a ceremonial ribbon cutting tomorrow and the start of bus service Sunday, Fairfax will join two other major suburbs, Montgomery County and Alexandria, as transit entrepreneurs. Montgomery pioneered the way 10 years ago with its Ride-On bus system. Alexandria launched its DASH bus service last year.

One other large suburb, Prince George's County, is weighing the feasibility of a similar bus venture. One smaller locality, Fairfax City, already runs a tiny bus system called CUE, slated for expansion next summer.

Advocates contend that locally run bus systems can sharply reduce costs, chiefly by paying lower wages than those earned by unionized Metrobus drivers. Such forecasts are questioned, however, by some officials who cite uncertainties about the financial results in Alexandria and Montgomery.

Proponents of local bus systems also argue that they can provide more grass-roots service, matched to neighborhood needs. They complain that Metro often gets bogged down in cumbersome procedures and squabbles among local governments.

Nevertheless, the trend has posed thorny problems for Metro and for local governments that lack bus systems of their own, such as the District of Columbia and Arlington County. Officials there warn of possible confusion for riders, fragmented planning and steeper costs for Metro service.

For Fairfax, embarking on the $2 million-a-year bus venture was not an easy decision. The plan has sparked protracted debate and three consultants' studies since the 1970s. Meanwhile, subsidies that the county pays for Metrobus service soared from $8.3 million in fiscal 1978 to $25.5 million last year.

Fairfax County Board Chairman John F. Herrity, a frequent Metro critic, termed the bus venture a first step that may eventually lead to eliminating all Metrobus service in the county.

"We will expand this from here," he said. "The burden's on us to produce now, and I think we will."

Fairfax Supervisor Joseph Alexander, a longtime Metro board member, demurred. "This is an experiment. I don't know whether it's good or bad," he said, predicting that Fairfax will retain Metrobus service on key routes for many years.

In addition to pinning their hopes on sizable savings, Fairfax officials sought to link their fledgling bus enterprise to the expanding Metrorail system, a strategy followed in Montgomery and Alexandria. All 10 Fairfax bus routes will serve the Huntington rail station, the county's only Metro stop.

If this first gamble pays off, Fairfax officials say, the county is likely to expand the local bus operation to tie in with the Metro subway system's Orange Line, which is scheduled to be extended to Vienna next summer.

Such prospects have stirred a wide-ranging debate among Metro and other officials, concerned about the outlook for the Metrobus system, a more than $230 million-a-year enterprise set up to replace the region's faltering, privately run bus companies. A blue-ribbon panel is currently weighing the issues.

Among key points in the debate:

*Will further proliferation of local bus systems lead to uncoordinated transit services, confusion for passengers and losses in ridership? Fares and other policies, such as whether to accept bus and subway transfers, vary among the systems.

*Will the District and other local governments get stuck with millions of dollars in long-term liabilities if some suburban governments withdraw from the Metrobus system? Officials are trying to calculate the costs of these "residual liabilities," including unfunded workers' compensation benefits, injury claims from bus and subway accidents, and unemployment and retirement benefits for former Metro employes.

*Will local governments that rely on Metrobus service have to shoulder an increasing share of Metro's overhead costs if other governments withdraw? It is unclear whether Metro can reduce these expenses at the same pace that suburban governments cut back on Metrobus service. Arlington, for example, will be hit with a higher share of Metro's "fixed costs" when Fairfax and Alexandria drop some of their Metrobus routes.

*Should Metro try to recoup some of its expenses by charging fees to local bus systems for using Metro facilities, such as bus stops at Metro subway stations? This issue has triggered heated debates and angry responses from some local governments. They have labeled the proposal counterproductive, saying it would discourage suburban efforts to bolster subway ridership.

*What should be the role of the Metrobus system? It was designed to consolidate bus service throughout the region as part of an overall transit network, linked to the expanding rail system. But the start of local bus systems has pointed toward a split: county and city sponsored systems would offer neighborhood service and "feeder" routes to rail stations, while Metro would provide long-distance or express bus service for commuters traveling across state, city or county lines.

*Will local systems achieve major long-term savings? The initial evidence is unclear, and some officials predict the savings will likely prove marginal. While local systems can reduce wages, overhead and other operating expenses, they also must incur substantial costs in buying buses and building garages. In addition, the local systems forfeit federal subsidies now provided to Metro. In Maryland, Montgomery also forgoes state aid for some Ride-On routes.

The 33-bus Fairfax system, expected to attract about 5,000 riders a day, has been designed as a carefully monitored experiment. It will take over 10 Metrobus routes in the southeastern part of the county, retain the same fares and transfer policies as those used by Metro, and make only slight changes in bus schedules.

"We wanted to make it as close as possible to what is now being operated by Metro," said Chris Jenks, who is overseeing the project. As a result, the county can readily compare the two systems and determine whether the predicted savings materialize.

The county has hired National Transit Services Inc., a Chicago-based company that manages 18 other transit systems nationwide, to run the Fairfax system under a $2.1 million contract. The move was aimed, in part, at cutting costs by avoiding the county's higher salary and benefit scale.

On the Fairfax Connector, drivers will start at $7.25 an hour, with a raise to $7.75 after six months. Alexandria drivers now get $7.50. In Montgomery, wages start at $7.73 and rise to $11.36 after 15 to 20 years. In contrast, Metrobus drivers start at $10.24 1/2 an hour and earn $13.66 after three years.

But other financial quirks may affect the local bus systems' bottom lines. Alexandria, for example, decided to reduce its fares to 50 cents. As a result, officials estimated, overall savings dropped to $200,000 in its first year, instead of the predicted $345,000.

Because Montgomery County forgoes a state subsidy on some Ride-On routes, officials said, it is unclear whether the county saves money by operating Ride-On instead of using Metrobuses.