MCI Communications Corp. will announce Tuesday that it will introduce a national low-cost electronic mail network to allow consumers to use their home computers to send messages via the phone lines and put the company into the growing data communications market.

Although MCI declined to comment on its plans, sources who have worked with MCI indicate that the company will use its network to carry data and high-quality facsimile/paper-replica communications as well as voice.

Essentially, the service will allow consumers and companies that own varying brands of personal computers, word-processors and facsimile machines to dial into the MCI network and transmit messages to the desired destination. A sophisticated computer system inside the network will mediate between the different brands of equipment so they can communicate with each other.

According to a report in the Chicago Sun-Times, MCI will price the service at $1 for computer-based messages roughly five pages in length that traverse the phone lines, with an additional $1 charge for each additional five pages. The data could be transmitted through the network "in a matter of minutes."

For those without communicating computers, the report said, a local MCI office reportedly will hand-deliver a paper message at rates ranging from $25 for four-hour delivery to $5 for overnight delivery. However, it is unclear how MCI would structure its paper-based service.

In essence, MCI apparently wants to transform itself from being just a low-cost long-distance voice communications provider into a low-cost provider of information communications as well. Several analysts suggest that telecommunications companies will have to broaden the array of services they offer as competition in the traditional telecommunications services intensifies. MCI has made announcements recently indicating that it would exploit the data communications market.

According to Stephen Chrust, a research analyst with Sanford C. Bernstein in New York, MCI has been "quietly testing" the service in several major markets and it will be "immediately available" after Tuesday's announcement.

"It's necessary over time to provide value-added services," asserted Chrust, "and this is certainly a large market and a high-volume business that's consistent with MCI's marketing efforts . . . "