Making its first move into the consumer market, GTE Telenet will announce this week a new electronic mail service targeted to owners of personal computers.

The Vienna-based subsidiary of General Telephone & Electronics plans to offer personal-computer owners a version of its GTE Telemail electronic mail service during off-peak hours for a flat fee of $25 a month. The service essentially allows computer users to distribute printed messages over phone lines.

Personal-computer owners who have a modem -- a device that enables computers to "talk" over phone lines -- will be able to use Telenet's nationwide network to send messages to other personal-computer users even if they are not subscribers to the service.

"I think it's a pretty innovative scheme to introduce micro users to the benefit of electronic mail," said William von Meister, a founder of The Source, a popular personal-computer network, and now chairman of Control Video Corp., a McLean-based personal-computer utility. "But I don't think they're going to get too much business at that price."

Von Meister pointed out that companies like The Source and Compuserve, an Ohio-based personal-computer network, offer comparable electronic mail services at a lower price. But he added that Telenet is in a position to price competitively because it is using off-hours capacity -- much in the same way AT&T cuts prices for long-distance calls made during off-peak hours.

GTE officials declined to discuss the service until it is officially announced.

Essentially, GTE Telenet is bidding to capture a share of the business from the estimated 500,000 personal-computer users with modems in the slowly evolving electronic mail market. A subscriber to the service could use a modem to call up another personal-computer user and send a computer-generated message of almost any length for the price of a local phone call. A special computer tone would signal the recipient to place the phone in the modem to get the message.

It is not clear whether GTE Telenet will provide central facilities that will store copies of messages for its off-peak users, as it currently does for its GTE Telemail customers.

With revenue estimated at nearly $100 million annually, GTE Telenet does not disclose its income, but says it is running in the black after several years of losses. The firm's major source of revenue is offering special computer-to-computer communications links for large corporations.