More than 120 large office buildings here are "unguarded these days because electronic security surveillance has come of age.

Keith Poore, a management executive with the Charles E. Smith Companies, said he has contracts with Kastle Sytems Inc. for guardless security at 40 buildings his firms owns.

"It simply works better and costs less than having live security guards on duty during weekends and off hours when some limited access must be provided to buildings to which tenants want to be admitted," he said.

Kastle's security system is controlled by computer out of a center at 1501 Wilson Blvd. in Rosslyn. The system allows authorized persons to enter the Kastle-guarded buildings by inserting a card into a small machine at the entrance.

If the card is authentic, electrical impulses to and from the monitoring center unlock the door quickly. After entrance, the closed door locks automatically.

But suppose you forget your card or expect a visitor?

A. Gene Samburg, the 38-year-old founder and president of Kastle Systems, says that 30 percent of the off-hour entrances are made by persons without cards. That's why a telephone is placed next to the card-reader station at the entrance.

The visitor identifies himself or herself by telephone to the person on "watch" at the monitoring station. If the name has not been submitted in advance for clearance, the monitor checks it out with te building tenant whose authorization is required.

In many cases, the card might be forgotten, Samburg said, and then the person's name checked on a master approved list via the telephone hookup.

In addition to the electronic admittance, Kastle also provides a sophisticated office-by-office security service for tenants. Keith Poore said that the admittance systems work well and have resulted in decreased thefts from buildings. Cleaning personnel have their own cards and are monitored as they work.

"Now almost no thefts occur after regular office hours," Poore said. He also said that tenants acceptance of the guardless security service has been good.

He said that the Smith firm does not charge tenants directly for the guard service. The initial investment for installation of the wiring system and mechanisms, which varies according to the size of the building (from $3,000 to $50,000) is usually amortized in less than five years. Even with the monthly service fee (ranging from $300 to $900), the Kastle system represents a savings, Poore added.

At his Kastle headquarters in Rosslyn, in a highrise building that uses the Kastle security system, Samburg presides over a 70-person organization and a lash-up of computer and electronic equipment.Security personnel keep a 24-hour watch on their clients and maintain a written record of all entrances and exits.

If an entrance card is voided or lost, the system can quickly be programmed to reject it, Samburg said.

An engineering graduate of Cornell University and a native of the Boston area, Samburg was with Westinghouse when he came up with the ideas for his system of "guardless" security for commecial buildings.

"I talked about it with my superiors, but they were not interested" he said. "So I decided to do it myself and got some financial backing and started with one building and kept growing."

James Crozier of Quadrangle Development Corp., a client with five Kastle-covered buildings, said that the system is reliable and economical. "It's difficult to get good guards." he said. "Of course, no system is perfect and there are some minor shortcomings."

Just what the shortcomings might be, neither Crozier, Poore, Samburg wanted to discuss for "obvious security reasons." But they agreed that even with some "gaps," the sytem provides more security than most guards. Poore said that the Smith firm supplements the computerized security system with its own night-time patrols in Crystal City, where many of the Smith office buildings are located. He said that Smith has a person on duty around the clock to monitor its own security and be in touch with Kastle control.

In event of a break-in or fire, a siren sounds. "A Kastle monitor duty person notifies police or firemen," Samburg said.

Although it is less than seven years old, the Kastle system is said by Samburg to be the largest of its kind, with total service covering more than 20 million square feet of office building space. He is planning to sell the service in other cities.

Samburg said that standard computers are used in the Kastle system and that the accompanying systems of security are designed by his staff and made under contract by Walter Kidde & Co. of Pawkatuck, Conn.

It takes two or three weeks to install the system in a small building and up to six weeks in a large building. Samburg said that there is a backlog of orders for installation.

One story Samburg tells customers when he is trying to sell them the system -- and he insists that it's true -- concerns his own market research: He says he once signed in at a building guarded by a watchman as "Donald Duck of Disneyland, Fla., and gained admittance without question."