The U.S. Postal Service will launch guaranteed, same-day delivery service in metropolitan areas early next year under a proposal approved yesterday by the agency's board of governors.

Reflecting a drive by Postmaster General William Bolger to build postal revenues by offering new services coupled with businesslike marketing, the "express mail metro service" would allow customers to bring letters or packages to designated post offices before 10 a.m. for delivery by 5 p.m.

The Postal Service will refund all postage if delivery is late and, for an extra fee, will pick up mail at a customer's premises on a scheduled basis.

Because the Postal Rate Commission must act on the proposed new service, the earliest that same-day delivery could start is April 1. Initially, the service will be offered in Chicago. Columbus and Gulfport, Miss.

Thereafter, same-day delivery would be started in Washington and in 18 other metropolitan areas by next October. In other postal developments yesterday:

The board of governors approved temporary implementation of an advanced electronic message service for large-volume mailers starting about Dec. 18. The electronic service was proposed to the rate commission on Sept. 8, and because there has been no decision, the Postal Service is free to begin operations pending formal hearings that open Feb. 5. An estimated 60 corporations will use the service in an initial 15-month period.

In the fiscal year ended last Sept. 30, the Postal Service deficit was $379.4 million, sharply below both an original forecast of $1.3 billion and the actual deficit of $688.8 million in the previous year. A record 96.5 billion pieces of mail were carried, productivity jumped 5.5 percent (mostly because of mail preparation by users) and a surplus of $180 million is forecast for the current year assuming inflation runs about 7 percent over the next nine months.

The Postal Service's proposed metropolitan area same-day delivery business would provide direct competition to private firms that have operated increasingly active delivery services in recent years, but the rate structure proposed is different from that of most private companies.

Three rates were proposed by the Postal Service: $9 for items weighing up to a pound, $12 for materials between a pound and 8 pounds, and $15 for pieces between 8 and 70 pounds. A charge of $5.25 would be made for pick-up service.

Most private delivery firms, in contrast, have a sliding scale of prices based on distance. Items delivered within a few blocks cost several dollars, while delivery between the city and suburbs often costs more than $10. Central Delivery Service of Washington charges $7.65 to deliver a package from downtown to Silver Spring during normal hours, for example.

Postal Service officials estimated potential same-day volume at 66 million pieces annually.