In the fullness of spring, the wine flows easily. A chilled glass of white for a Mother's Day toast. A red with an elegant dinner. Wine comes from grapes, of course, but it's easy to forget that wine-making begins as an agricultural pursuit.

Archie Smith III knows. He is the 33-year-old winemaker at Meredyth Vineyards, a family-owned farm in Middleburg, Va. A month ago, he was far more concerned with the weather than how any particular wine tasted or the latest critical appraisal of Bertrand Russell's works. (Smith has a Ph. D in philosophy from Oxford University). He was sweating out that dangerous period when the buds come out and there still is a danger of frost. Late frost devastated vineyards in the Loire Valley of France in 1977 and again in 1978. It strikes elsewhere, too, killing the infant buds and cutting the potential crop. Growers lose money and the lack of wine forces prices up. Everyone is hurt.

In middleburg, bud-break, as it is called, usually comes about April 15. Seldom is there frost after April 21. This year, he made it. No sudden warm spell pushed out the buds prematurely. There were cold nights in April, but no frosts.

On the 15th, a Saturday, there was intermittent sun and a chill wind. Smith led some visitors through the 35-acre vineyard. The fall harvest and crush receive all the attention from writers and poets. But even as the vines lie dormant through the winter, work goes on. Pruning them, cutting back the arms or shoots developed during the season past, is done to concentrate the sap that will begin to flow in the spring. Old vines and new must be checked and tied to the wires and posts that support them.

By April the vineyard is ready for bud-break. The vines are gray, gnarled and sad-looking. The wooden posts are weathered and drab. The ground under them is brown, with only a few stray sprigs of green to relieve the monotony. The romance of wine couldn't be further away, either chronologically or in mood.

But Archie Smith knows what's coming and tells us that the vines already have begun to swell with sap. Words spilled over one another as he projected the pattern of the next few weeks. "Bud break is beautiful," he said, pointing to some tiny, knotlike bumps that still tightly closed. "All of a sudden you have a pale green cloud of tiny leaves floating six feet off the ground. It's very, very pretty." If all goes well, bloom, or flowering, will follow in four to six weeks.

In the interval, Smith will do what he can to nurture the crop. The ground will be torn up with discs to prevent weeds from competing with the vines for moisture and nutrients. Later, he will plant a ground crop to prevent excessive vine growth.

There are a dozen different grapes growing at Meredyth, some varietals but the bulk hybrids. Each has its own personality. That compounds the task of caring for the vineyard, as does the rolling ground and the presence of a creek nearby. They cause minute but significant differences in the climate. Then there is the problem of bug and disease control.

From bud-break on, inspecting the vineyards is a daily chore. Add to it, testing the wines aging in vats and barrel, doing paperwork and inventory control for the government and trying to promote sales, and you will discover why farmers, even wine farmers, go to bed early at night. They're tired.

There is always time to taste wine, though. Among, those Smith presented to the visitors were a delicate 1978 riesling, a hearty 1976 villard noir that matches well with roast beef or cheeses, and a 1978 rougeon rose that will make a delightful picnic or aperitif wine this summer.

Meredyth wines at the vineyard are $4 a bottle. They are also sold at several District and Northern Virginia stores.