The next time you need a vacationing physician's advice in an emergency, a national paging service could come to the rescue.

Washington-based MCI Corp., American Express Co., and five firms that operate radio paging systems announced plans yesterday for a nationwide network to provide electronic-message delivery service.

The new company, not yet named, would ultimately provide service in 200 metropolitan areas through the use of MCI's existing long-distance network and the paging facilities operated by Metromedia Inc., ICS Communications, Communications Industries, Zip-Call and MCI Airsignal.

In a joint statement, the seven companies said they were actively pursuing agreements with other paging companies to interconnect both large and small metropolitan areas.

This was the second such venture announced this month. On July 14, National Public Radio announced it was teaming up with Mobile Communications Corp. of America, fourth-largest of the nation's 700 radio-paging systems, to form a national system that would operate through NPR's satellite network.

Both nationwide systems would operate in the 900 megahertz frequency, pending approval by the Federal Communications Commission. The FCC voted in April to open new frequencies for paging in the 900 megahertz band and will begin accepting applications Sept. 7.

The companies that announced the joint venture yesterday would invest between $15 million and $20 million in the national network.

The paging service may also be available on a limited international basis within a year, according to MCI Chairman William C. McGowan.