In a densely populated county where living space is usually measured in small lots and tightly arranged apartments and condominiums, the vast Rancho Mission Viejo might seem a mythical place in a dime Western.

The ranch sprawls over 40,000 acres of amber hills and grasslands that is pure cattle country. Since 1882, cowboys have ridden the brand of the Rancho Mission Viejo, repairing fences and rounding up cattle for market.

Although the inexorable tide of development wipes out much of Orange County's pioneer heritage, family members say the last roundup at the rancho will not be anytime soon.

Patriarch Richard O'Neill Sr. bought the ranch more than a century ago. In recent decades, real estate interests have come to the O'Neills with proposals to build everything from an amusement park to a garbage dump on the ranch property.

One of the most influential families in southern Orange County, the O'Neills own the Santa Margarita Co., builder of Rancho Santa Margarita, a planned community.

Although a number in the clan are major land developers in the county, when it comes to building on the family ranch, their answer has been a flat "No."

"All these people have designs on the land," said Gilbert Aguirre, who runs Rancho Mission Viejo. "Our world is getting smaller, but the family is dedicated to being in the cattle business for the next 110 years."

The family also has strong links to the historic mission town of San Juan Capistrano. Dick O'Neill, 70, a grandson of the ranch founder, is trying to buy and redevelop a block of downtown commercial property there. O'Neill, who owns the El Adobe restaurant in town, has been heavily involved with the Orange County Democratic Party over the years. The O'Neill family's wealth was estimated at $500 million by Forbes magazine in 1992. Both the ranch and the city share a rough-and-tumble legacy that lasted through the 1950s, when old-timers say it was not uncommon to see cowboys riding their horses through the saloons.

"Those were golden times," said Tony Moiso, the grandson of Richard O'Neill who oversees ranch operations. "San Juan Capistrano was so isolated in those days before (Interstate 5) was built in the early '60s. We feel very much tied to the town."

At one point, 15,000 head of cattle were raised at the Rancho Mission Viejo. Today, about 6,000 head graze there, still making it the largest cattle operation in Southern California, said local historian Jim Sleeper.

But in recent years, development has whittled down the amount of prime grassland available for cattle and even the Rancho Mission Viejo has been forced to buy acreage in northern Nevada, where the herd is sent to feed each winter.

Although beef prices have been good for the last few years, "historically, it's not been very profitable to raise cattle for most people," Aguirre said. "You can't control the prices set on your product."

To generate income to run the ranch--which is about three times the size of San Francisco--the family, headed by Moiso, began planning the community of Rancho Santa Margarita at the foot of the Saddleback Mountains in 1983.

Angered over development of the pristine canyon area, the plans drew heavy criticism from residents and environmentalists. But it was approved, and today 20,000 residents live in the half-completed development. Meanwhile, company officials established a 1,600-acre nature conservancy on the ranch that is open to public tours.

"We could throw a fence around the place and keep everyone out, but that would be terribly irresponsible," Moiso said. "I feel blessed to be able to develop a portion of the land and remain a rancher on the rest."

However, local environmentalist Sherry Meddick is torn about the Santa Margarita Co.'s environmental record.

"I've always felt so funny that someone could donate incredible resources and also be responsible for the development coming behind it that impacts these preserves," said Meddick, a Silverado Canyon resident who works for Greenpeace in Los Angeles. "They donate the parkland but unfortunately, the flood plain from the (housing) developments flows into the park area.

"I frankly think Dick and Donna O'Neill have concerns for the land . . . but if the area's ecological integrity is disturbed, I don't see the point of donating" parkland.

The family patriarch, Richard O'Neill, was a former butcher who, after arriving in the United States from Ireland, operated a ranch in Merced County. When the 200,000-acre estate of John Forster was put up for sale, O'Neill saw an opportunity he could not pass up.

It was truly the big country. Stretching from Oceanside to Aliso Creek in what is now Lake Forest, O'Neill was looking at 359 square miles of prime grazing territory.

He found a financier in silver magnate James Flood, who was willing to put up $457,000. In 1882, the men purchased the vast plot of land. O'Neill lived another 24 years, surviving to see the Santa Fe Railroad lay tracks through the ranch, greatly increasing its worth.

The ranch passed through the hands of three generations of O'Neills. The immense property was divided in the late 1930s when the Flood heirs decided to leave the cattle business.

The O'Neills kept the Orange County portion of the ranch, a decision that soon proved prescient when the federal government in 1942 enacted the War Powers Act and, through its eminent domain powers, took over what became Camp Pendleton, a training base for the Marines.

The family started the planned community of Mission Viejo in the mid-1960s, but sold the 10,000-acre development in 1972 to the Phillip Morris Co.

Throughout their dealings, the family has been determined to remain in the cattle business.

The annual spring roundup, when calves are born and branded, remains a deeply ingrained ranch tradition. The El Viaje de Portola, where several hundred riders re-create the trail of Spanish explorer Gaspar de Portola in southern Orange County, is another way the family celebrates the area's heritage.