Tourists are swarming the area around Ronald Reagan's Santa Barbara County ranch this summer. They have come, however, not to visit Rancho del Cielo or the nation's first family. They have come to tour and taste in California's newest wine-producing area.

The Santa Ynez Valley, long known for beef cattle and thoroughbred horses, is on the brink of breaking into the big leagues of California winemaking. Designation of the appellation "Santa Ynez Valley" as California's newest viticultural area was made official by the federal government on May 16.

Producers such as Firestone Vineyard, Zaca Mesa Winery and Santa Ynez Valley Winery have already won numerous medals for their recent vintages and have established reputations beyond the Golden State. New wineries, like Brander--its wines served this spring at the Williamsburg Summit--are also beginning to attract national attention. Even small, local wineries such as J. Carey Cellars, Ballard Canyon Winery, Vega Vineyards and Austin Cellars confirm the seriousness of wine production in the area and signal the potential for growth despite the economic maladies that beset the California wine industry these days.

As in California's better known wine regions, grapes first were grown in the valley by 19th-century Spanish missionaries and a few pre-Prohibition farmers. But it took the wine explosion of the early 1970s to develop widespread interest here in the glories of grapes.

The modern era in the Santa Ynez Valley Winery began in 1969, when mainly cabernet sauvignon vines were planted in the western foothills of the valley. While the grapes were sold to far away Paul Masson Vineyards in those early years, native Frederick Brander began making experimental wines in 1974 for the fledgling Santa Ynez Valley Winery. By 1977, Brander had captured a gold medal for a white wine (sauvignon blanc) from grapes grown at his own small vineyard at Los Olivos, 10 miles to the east.

Successive bottlings of Santa Ynez Valley Winery sauvignon blanc have been equally impressive. The 1981 reserve has a stalky, varietal nose and clean, grassy flavor. It is more balanced and has a rounder finish than the tart, acidic 1981 reserve chardonnay.

Brander, who will relinquish some of his winemaking duties at Santa Ynez Valley Winery this fall, is developing a more austere sauvignon blanc at his own, newly opened winery. From vineyards that get more heat and less fog than those surrounding the former dairy farm that has become the Santa Ynez Valley Winery, Brander will produce some 5,000 cases this year--principally sauvignon blanc. Made in the graves tradition of Bordeaux, the 1981 and 1982 wines will be appreciated only by devotees of lean, high-acid sauvignon blanc. Stylistic extremes notwithstanding, the 31-year-old Brander is adamant about restraining costs and plans to market his wines aggressively at prices between $7 and $9 .

While Santa Ynez Valley Winery may claim the oldest vineyards south of the old Nielsen vineyards (c. 1964) in nearby Santa Maria, the oldest winery in the valley is The Firestone Vineyard--from the same Ohio farming family that gave America the radial tire.

"My father had bought vineyard land in the valley in 1972 as an investment," explains Brooks Firestone, whose study of the wine business included a stint with Chappellet Vineyards in Napa Valley before he built a winery and presided over the first small crush of grapes in 1975. Seven vintages later, Firestone is now making very good wines at reasonable prices. Watching costs has enabled the winery--now producing 70,000 cases a year--to post its first profits. Notes Firestone, "Twelve years in the tire business teaches you to be frugal."

The Firestone reputation has been built on successive vintages of a spicy, well-balanced chardonnay and, recently, on a luscious, honeyed late-harvest riesling that is very reasonably priced. The 1981 chardonnay--served to England's Queen Elizabeth II on her recent visit to California--is one of the finest medium-bodied chardonnays available. It is a real bargain when discounted to $9.

And while 50 acres of zinfandel grapes have been plowed under, the Firestone cabernet sauvignon that replaced them merits close monitoring. The 1977 Private Reserve Cabernet, from the Ambassador vineyards near the modern, angular winery, is a big, textured wine with layers of flavor that should develop into an elegant, stylish wine at half the cost of other California reserve cabernets.

Somewhat less successful have been wines from the Arroyo Perdido vineyard, surrounding the Firestone family's ranch-style home. Its unique soil, unlike the gravel and limestone surrounding the winery, has produced controversial wines and, consequently, the vineyard is being phased out.

At Zaca Mesa Winery up the road, a major expansion is under way. Earlier this month, Zaca Mesa consummated the acquisition of Los Vineros Winery in nearby Santa Maria as part of a multi-million dollar expansion program by which it intends to become one of the Central Coast's largest producers. "We've been growing by a factor of 50 percent a year since our winery building was completed in 1978," says president Marshall Ream, a retired Arco executive, who adds that the anticipated growth will not be achieved at the price of quality or affordability. Zaca Mesa's 1983 production will top 80,000 cases.

Ream, who bought a ranch here in 1973 as a summer getaway, believes that the Los Vineros acquisition will enable Zaca Mesa "within a few years" to grow into a 200,000-case winery. Expansion of its 220-acre vineyards surrounding the present winery was foreclosed because of restrictive zoning laws in environmentally sensitive Santa Barbara County.

The Zaca Mesa reputation was boosted last year with release of a moderately priced 1980 chardonnay with round, spicy nutmeg flavors and a full yet balanced finish. The 1981 chardonnay is somewhat thinner. The winery's pinot noir and cabernet sauvignon, while not complex wines, represent good value at less than $8 a bottle. A Toyon line of inexpensive, cork-finished table wines was introduced last year to complement its regular wines and a small production of American Reserve bottlings.

Retail sales in its tasting room and wine boutique exceed 10 percent of Zaca Mesa's total production, boosted no doubt by the constant stream of tourist traffic along U.S. 101, some 150 miles north of Los Angeles.

But Santa Ynez Valley producers are looking beyond the relative proximity of the Los Angeles market in their quest for acceptance as one of the state's major wine producing regions. With a somewhat cooler climate and definitely lower costs than areas like Napa Valley, entrepreneurs in this valley are gambling that the currently saturated California wine market can still support quality producers with an eye on value to the consumer.

"It's hard to describe our new region because different wineries are going in their own directions," says Firestone. "We did the studies and it looks good. But to some extent," he adds, "it's a roll of the dice."

Having surprised the once-skeptical cattle ranchers, Ream is nevertheless restrained about his initial success. "Too many people bring pomposity to wine production," he says. "They think their wine a gift from the gods."

To these California farmers, tending vineyards just a short horseback ride from President Reagan's ranch, producing great wine may be a special art. But they also know that the sun is setting on the show-biz days of the 1970s. As well as any of California's 400 producers, they understand that surviving in these value-conscious years of the 1980s is a complicated, risky business. Wine Notes

* Christie's next U.S. auction--Oct. 15 in Chicago; catalogues $10 by mail from Christie's, 46 E. Elm St., Chicago, Ill. 60611.

* Summer thirst for wine knowledge? August wine courses at Wide World of Wines, 4801 Massachusetts Ave., NW, Washington, D.C. 737-9463.

* Paris bound? Great vintages at great franc prices at the Nicolas outlet on rue Faubourg St. Honore', near the American embassy.