Advanced Technology Inc., a fast-growing professional services firm in Reston, yesterday said it has been sold for $140 million to Emhart Corp., a $3 billion-a-year manufacturing and information services firm.

ATI is a privately owned firm that has grown from a five-person operation with $1.4 million in revenue in 1977 to a $153 million-a-year company with 2,300 employes across the country.

Its five founders -- Bob LaRose, president and chief executive officer; Ron Hobbs, executive vice president, and senior vice presidents Tony St. George, Bob Darby and Girish Jindia -- all became multimillionaires yesterday as majority shareholders.

About 40 percent of the stock is owned by an employe stock ownership plan, Hobbs said. The price -- to be paid in cash and Emhart stock -- represents about $39 a share for the outstanding shares. ATI estimated that the shares had a book value of about $5.38 per share in its 1987 annual report.

The purchase is the second move into the Washington professional services field by Emhart, which last year purchased Planning Research Corp. of McLean for $210 million in cash.

PRC was the first venture in the information and technology area for Emhart, which over the last two years has sold 15 of its manufacturing businesses and sought to build its information businesses.

"PRC was the nucleus of that business," said a spokesman for Emhart yesterday. "ATI is a major building block ... with talent and an enormously successful track record."

For ATI, which will continue to operate under its present management as a subsidiary of Emhart, the new ownership "can help us significantly," said executive vice president Hobbs. "We're looking for opportunities to bid for some very large jobs. The strategic alliance was the key thing."

"There's a tremendous euphoria in the air here," said Hobbs. "We're extremely excited."

ATI reported $38 million in revenue in its most recent quarter, with $3 million in pretax profits.

The chief asset of companies like ATI is the "brain trust" of the firm's employes, many of them high-technology experts with advanced degrees.

The firm recently announced $63 million in new contracts including work for the Navy, the Office of Personnel Management, the Tennessee Valley Authority and the Health Care Financing Administration.

Emhart's acquisition of ATI, following its PRC purchase, is expected to add to the ability of both companies to gain government contracts.

The two, which have frequently competed for major government contracts, will now work closely on many projects, Emhart officials said.

ATI will especially complement PRC's bidding for Navy contracts, analysts said.

More than 50 percent of ATI's business is with the Navy, including contracts involving communications and radar on the new Aegis cruisers and destroyers.

Financial analysts said the price was competitive for the industry.

"It's a high-price industry because of the growth rates," said Wilhelm Merck of Adams, Harkness & Hill, a Boston securities firm.

Emhart, based in Farmington, Conn., lost $10.2 million on revenue of $2.9 billion last year after a $90 million after-tax restructuring charge.

Since 1985, the company's president, Peter L. Scott, a former manager of United Technologies' $3 billion electronics business, has taken Emhart in a new direction.

In the first six months of 1987, Emhart had record earnings of $56.8 million on revenue of $1.2 billion. Emhart is number 170 in Fortune magazine's 1987 ranking of the nation's 500 largest industrial corporations. Its shares are traded on the New York Stock Exchange.