Negotiations are under way for Toyota, the Japanese automaker, to assume sponsorship of the Colgate Series women's pro tennis tour, it was learned last night. Souces said a change of sponsor also might result in a move of the series championships from the Washington area.

A source close to the Colgate Series Championships said the proposal to change sponsors was discussed at a meeting of tournament directors yesterday and that it is "virtually done."

John Beddington, vice president of the International Management Group, which sponsors several circuit events, and who was present at the meeting, said, "I can confirm that discussions are taking place with a potential sponsor for the nine-month women's circuit currently sponsored by Colgate. But nothing has been finalized yet."

Asked whether it was a virtual certainty, he said, "The discussions that have taken place are certainly of an extremely serious nature but there is still a lot more negotiating to be done before anything can be finalized."

Such a proposal would have to be ratified by the Women's Tennis Association, as well as the Women's International Professional Tennis Council (WIPTC). The pro council meets here Monday.

The current Colgate contract extends through the end of the 1981 season, a WTA official said.

There has been speculation, in recent days, that the date of the Colgate Series Championships, the tournament that supposedly decides "who's number 1" for the year, might be moved up to December.

A source close to the tournament said, "A sponsor in negotiation with the pro council might have some leverage in when it wants the finals played."

The source said that several sites were being discussed for next year's championships, including Washington, Tokyo, Los Angeles and the Meadowlands in New Jersey.

Beddington said, "Some thoughts were put forward about changing the date and venue," but stressed that "nothing has been decided yet."

Ray Benton, tournament director, said, "I'm not concerned about it. Washington has had one of the premier events every year for the last 10 years. Avon wants to come back in. Washington has proved itself to be one of the strongest supporters of women's tennis. There's no way we won't have a major event here next year."