The Washington Post Co. announced yesterday that it has agreed to purchase Legi-Slate Inc., a small electronic data analysis firm that provides computerized information on congressional legislation and federal regulatory activities.

Post President Richard Simmons said the company would not reveal the price it expects to pay for the firm. Legi-Slate was founded in 1978 to track legislation and voting records in the Texas legislature.

The company began tracking Congress and regulatory agencies in 1980, according to its president and founder, Curtiss Grove. Grove will continue to run the company, which plans to expand its monitoring activities to all 50 state legislatures.

Legi-Slate tracks every bill and resolution in Congress and provides subscribers daily updates on the status of legislation, all committee and subcommittee actions, floor votes and the voting record of every senator and representative.

Grove said Legi-Slate's subscribers include large companies, federal agencies, trade associations, labor unions, law firms, political consultants and embassies.

Both Simmons and Grove said that at some point in the future, Legi-Slate will be able to offer subscribers access to articles that appear in either The Washington Post or Newsweek--both owned by the Post Co.--while Legi-Slate might prepare material for either the newspaper, the magazine or for Newsweek's cable television programming service.