One Sunday not long ago, in the valley town where she has spent most of her life, Marilouise Kornell drove away from mass and spent half an hour trying to get across Main Street.

The traffic would not let up. Not a single break appeared in the bumper-to-bumper convoy of station wagons and motorcyles and Air Stream trailers. Kornerll, a third generation Napa Valley vintner, turned off her car engine and settled back to wait.

By the carload, by the busful, they were comings to taste the wine. They had started at one end of the valley - a narrow swath of vineyard and wineries just northeast of San Francisco as lovely as any place in California - and it is likely that they were drinking their way to the other end. Last year about 2 million visitors funneled into Napa Valley to tour the wineries and sample their products, and for Kornell and a number of other vintners, the whole thing is getting a little out of hand.

Last month, in preliminary report to the Napa Valley Vintners Association, a group of winemakers made the apparently unprecedented suggestion that the wineries try charging admission fees, or offering tours by appointment only.

Somehow, the report said, the visiting masses have to be cut back, because what began as a congenial introduction to fine wines has developed into a tourist attraction of such monster proportions that, if it keeps up, said association head Jack Davies, "it will ruin and destroy what everybody's coming to see."

The traffic jams are monumental. Cars sit for 20 minutes, spewing exhaust, while waiting for a chance to turn left into a winery parking lot. Campers pull off onto private property, and picnickers plop down in vineyards.

And now, on land once cultivated with such reverence for tradition that only the hillside grapes - considered the best - were used for sacramental wine, Kornell sometimes finds bags of garbage dumpedd of by drivers of mofor homes.

A hundred years ago immigrants lie Kornell's Italian-Swiss grandfather found in this country soil that would grow grapes and a lush cleft in the mountains that reminded them of Europe. They planted cuttings carried from the homeland and nurtured the grapes to wine.

The vineyards now are famous, nationally for producing wines that impress even France, and locally for providing visitors with a now firmly established California tradition referred to as the Day in the Wine Country.

At its most discreet this consists of touring a small winery, by appointment, and perhaps buying a case of particularly fine gray Riesling to bring home. There are variations, however. From the first big welcome sign to the last, a casual visitor can pull into a dozen hospitable wineries without ever straying far from the main road. The idea is to taste, but guzzlers abound.

"people just come up here to get juiced," a local bartender said. The Kornell champagne cellar is near the north end of the valley, and, as Kornell's daughter, Paula, said, "You can always tell when they've started at the other end. They're feeling no pain."

The Napa Valley tastings had their beginnings in 1934, after prohibition had been repealed and an energetic Napa Valley vintner, celebrating the end of making prunes-in-wine and apricots-in-sherry, decided to reintroduce the public to his product by holding multi-course dinners that featured his wines. The vintner would explain: red with meat, white with fish, and so on.