The Discovery Channel, the Bethesda-based cable network that's turned educational programming into an entertainment gold mine, plans to add the Learning Channel to its fast-growing broadcast empire for a sum that could exceed $25.5 million.

Discovery officials announced yesterday that they will pay $12.75 million to buy the 51 percent stake in the Learning Channel now owned by financially troubled Financial News Network and Infotechnology Inc. Discovery will pay an additional, undisclosed sum that sources say could exceed $12.75 million to buy the remaining 49 percent stake that is held by the nonprofit American Community Service Network and by the Learning Channel's managers and employees.

The purchase gives Discovery a separate cable network that provides educational programming to 20 million homes and thousands of schools and colleges. It fits neatly with Discovery's mission of providing educational but entertaining viewing fare, officials from both companies said.

Discovery has grown to more than 54 million subscribers since it began broadcasting full-time, nonfiction programming to 156,000 households more than five years ago -- an expansion rate that outpaced even MTV during its first five years. Its popularity has been built on shows such as "Arabia: Sand, Sea & Sky," a three-hour documentary on the Arabian peninsula.

The sale of the Learning Channel ends months of uncertainty about the fate of the Rosslyn-based education cable channel and comes just two days after FNN agreed to be sold to a joint venture of Dow Jones & Co. and Westinghouse Electric Corp. for an estimated $90 million.

Discovery employs about 200 people, the Learning Channel about 40.

The proposed sale is expected to be completed by May 1.