The Washington Post Co. yesterday announced preliminary plans to purchase interests in two pay television sports networks, including a New York service that already has long-term contracts with the New York Mets and New York Yankees baseball teams.

The plans involve New York's SportsChannel and the New England SportsChannel, a similar pay television service in the Boston region that carries Boston Celtics basketball and Hartford Whalers hockey. The Post plans to purchase about 50 percent of the New York network and about 25 percent of the New England venture.

The agreement in principle was announced by Richard D. Simmons, president of the Post Co., and Charles F. Dolan, the managing general partner of Cablevision Program Enterprises, a general partner in the sports ventures. Financial details of the proposed transaction were not disclosed.

Ventures controlled by Dolan, a long-time cable television entrepreneur and one of Home Box Office's originators, provide cable service on Long Island and recently won franchise awards for all of the Bronx and about two-thirds of Brooklyn. In addition, other Dolan ventures have won cable franchises in Boston and suburban Chicago.

The decision to move into pay television sports is the first major cable television move by the Post Co. "This venture is a splendid opportunity for our company to enter a cable programming business with exceptional potential for profit growth," Simmons said.

The New York service is purchased by about 270,000 cable televison subscribers and more than 100,000 subscription television viewers. Manhattan Cable TV, for instance, a service that reaches about 170,000 Manhattan homes and businesses, typically charges most subscribers $9.95 to hook up the service and $9.50 a month to subscribe. Those charges vary, however, depending on whether the subscriber buys other services.

The network has 30-year contracts with the Mets and hockey's New York Islanders, a 15-year contract with the Yankees and pacts with the New Jersey Devils hockey team, basketball's New Jersey Nets, and with the New York Racing Association.

The baseball games shown by SportsChannel are not shown by advertiser-supported, over-the-air television stations.