THEY RARELY MAKE PUBLIC APPEARANCES. THEY DON'T DO much business with Washington-area companies. They seem to be content living out of the limelight and making lots of money. On all levels -- personal, professional, civic -- the three children of Forrest Mars Sr. preach simplicity, frugality, privacy. And by most accounts they practice what they preach.

For 22 years, John Mars and his wife, Adrienne, have lived in the same house in the Northern Virginia suburbs. Their two-story colonial is subdued compared with the other upper-middle-class homes nearby. With its cedar shakes and wood siding, it sports a weathered look. On occasion, the roof leaks. In the spring, Adrienne can be seen on the front lawn, clearing the weeds from beneath the bushes or planting new bulbs in the flower garden. If John's home, he'll cut the grass.

Forrest Mars Jr. recently purchased a $1 million condo in an upscale suburban high-rise -- but there is little else about his life that hints at the true extent of his wealth. He's not a stylish dresser. His 12-year-old blue Mercedes was a gift from his mother.

Forrest Jr. is four years older than his brother, but, friends say, he acts much younger. Shortly after the last of his four children left the house, he divorced his wife of 33 years, Virginia, and married a co-worker roughly half his age. At 60, he's recently taken up scuba diving. He also loves to fish and is an expert skier.

Mars Inc. general counsel and family friend Edward Stegemann says that John and Forrest Jr. have no concept of their riches. "Deep down, they believe they are poor. They want to be normal, middle-class Americans."

"If you've figured out a way to make a living without working," John Mars told me with a completely straight face, "I'd certainly like to know."

Jacqueline Mars has stayed even farther out of the spotlight. On paper, she's an equal partner with her brothers and a company director, but she hasn't taken much part in day-to-day management. Friends say it's because Jackie, although extremely bright and capable, has never been encouraged to play an active role. At 52, she has only recently been named to the management team as corporate vice president. Like her older brothers, the Bryn Mawr graduate lives an unassuming life. A personal shopper who doggedly pursued the billionaire a couple of years ago says she was delighted when Jackie proclaimed that she'd pay her a commission to buy her clothes. But the excitement vanished when Jackie added a catch: "She wanted me to shop at Sears, not Saks," the shopper recalls. "At first, I thought she was joking. But the joke was on me."

Although Jackie has a desk at Mars headquarters in McLean, she spends most of her time working for the Mars Foundation, a tax-exempt charitable organization that is supported by the business. In 1990, Mars contributed $750,000 to the foundation, which in turn donated money to various family interests -- $9,000 to the Hotchkiss preparatory school, John and Forrest Jr.'s first alma mater, for example; $5,000 to Yale; and $7,000 to the American Cancer Society. (The family also provides support to the National Symphony Orchestra and the Kennedy Center.)

In total, the foundation passed out less than $1 million in 1990, a sum critics call small given the family's vast resources. According to Washington business leader Sheldon "Bud" Fantle, Mars Inc. has declined invitations to join the United Way Campaign or the Greater Washington Board of Trade, the area's largest chamber of commerce, much to the chagrin of the Washington business establishment. "It's unbelievable the way they carry on over there," says Fantle, former chairman of the Peoples Drug Store chain. "We've asked them over and over to help make a difference in this community, and they've refused every time."

When Joe L. Allbritton, chairman of Riggs National Bank, called on the McLean headquarters several years back to drum up business -- he wanted Mars to open a corporate account -- he couldn't even get John Mars to talk to him. Allbritton did wheedle his way inside to meet John in person, but after the banker made his pitch, John just stared at him, never saying a word. Allbritton told colleagues it was the strangest meeting he'd ever attended. Needless to say, he never got any business from the family.

When Mars was asked to invest in Washington baseball, Forrest Jr. turned down the offer.

Are the Marses stingy? No one really knows, but the company's accountants don't think so. In fact, they say that in the last decade, the company has donated tens of millions of dollars to charities worldwide. These anonymous donations, which have included funds for a local hospice and airplanes for doctors in the Australian Outback, were made by the company, not the Mars Foundation. But the family doesn't want anyone to know about this, and Mars officially denies making such contributions out of fear the acknowledgment will spark a flood of requests.

And what of Forrest Mars Sr.? He still lives in an apartment above the Ethel M candy factory in Las Vegas and pays rent to his sons. At 87, he's slowing down a little and no longer watches over the factory's day-to-day activity. In fact, he's turned his attention away from pet food and candy making. He's researching a new enterprise: plant food.