The regional Bell telephone companies and AT&T won important concessions yesterday when the Federal Communications Commission allowed them to offer new computerized services such as burglar alarm or health-monitoring services without having to use separate subsidiaries.

Experts said the new rules open opportunities for joint ventures between Bell operating companies and computer makers that may be worth hundreds of millions of dollars.

But the FCC decision also drew criticism from persons who said that the Bell companies now more easily could use revenue from residential telephone customers to subsidize the ventures and could use the new authority to crush would-be competitors.

"We've done a lot to eliminate costly inefficient standards," FCC Chairman Mark S. Fowler said. "We anticipate dynamic technological change. . . . All of this for just pennies a day; all of this is important because of the effect on the common man."

Fowler said the removal of current restrictions "can significantly lower local telephone costs" by spreading the costs of the telephone network over a larger base of customers.

But the agreement that governs the Bell System breakup still impedes the entry of the companies into new computer areas, he said and added: "This obstruction must be removed in the public's interest."

These new FCC rules will have no real teeth until U.S. District Judge Harold H. Greene, who presided over the breakup of the Bell System, lifts restrictions prohibiting Bell operating companies from providing computerized services and barring AT&T from offering data-processing services through anything but a separate subsidiary. Greene will continue to judge requests to enter new lines of business one at a time.

Even with the FCC decision, the regional phone companies will have to apply to Greene for permission for each new venture they want to pursue.

The FCC lifted the requirement for separate subsidiaries to handle new ventures because it said the requirement was cumbersome and discouraged new services. FCC officials said that rate payers and competitors will be protected by new rules.

In a related action, the FCC's temporary permission to AT&T to offer data-processing services through other units besides its Information Services Division also was made permanent.

Bell Atlantic Corp., the regional telephone company that owns the Chesapeake & Potomac Telephone Cos., said the decision means high-technology services such as computerized message-taking will be affordable for the average consumer. New services such as monitoring heartbeats over telephone lines or delivering voice messages forwarded by a network computer hours after they were recorded are on the horizon.

Computer companies have been keeping a watchful eye on Bell companies that market business equipment. John Sculley, chairman of Apple Computer Co., said, "I believe they are going to be significant factors long term in the computer industry. . . . It would be Apple's strategy to be part of that."

But others criticized the commission for lifting restrictions without having solid new protections in place.

"For years at the FCC, the reliance upon accounting mechanisms was considered a totally inadequate way of monitoring subsidies," said Charles Ferris, a lawyer and former FCC chairman. "As long as the Bell operating companies are the only game in town for local phone service, the risk of subsidizing the other ventures of the local phone companies, I think, is very great."

On Capitol Hill, where the debate to relieve the Bell companies of restrictions has been intensifying, Rep. Ron Wyden (D-Ore.), a member of the House subcommittee on telecommunications, said, "I don't know how you can impose an accounting mechanism when you don't know the costs of the phone industry -- and they have no idea.

"I have real questions about whether the consumer is getting the short end of the stick again."