The Heritage Foundation has been using a taped endorsement from President Reagan, apparently without his knowledge, to raise money.

The conservative research organization, which earlier this year solicited contributions with the help of a letter from White House counselor Edwin Meese III, recently sent letters inviting 35,000 former donors to dial a toll-free number and hear a taped message in which Reagan praised the foundation's work. This was followed by an appeal for contributions by Edwin J. Feulner Jr., who heads the tax-exempt group.

Reagan said on the tape that, after he was elected, "There was one group that gave us special, substantive help we'll never forget. I'm talking, of course, about that feisty new kid on the conservative block, the Heritage Foundation."

Reagan said the foundation's 1,093-page transition report "combined mastery of the federal bureaucracy with a no-nonsense plan to cut it down to size. We've been using it to our and the country's advantage ever since....As a matter of fact, one of the people it's been most useful to, and used by, is me."

The next voice on the tape is Feulner's. "I'm pleased that President Reagan thinks so highly of our organization. I hope you will renew your financial support today. We urgently need your help if we're going to continue to provide the Reagan administration with the kind of hard-hitting research President Reagan described in his message."

White House spokesman Anson Franklin said this week that Reagan taped the message "out of gratitude" for the foundation's help, but that "it was not prepared for fund-raising purposes. The president did not know it would be used in connection with a fund-raising letter."

Anson said the White House does not plan to object to the practice. "We do not have a reaction one way or another," he said.

Herb Berkowitz, a spokesman for the Heritage Foundation, said he believed the White House knew that Reagan's message would be used in part to raise money. He said the president originally made the comments last summer for a 14-minute promotional film on the foundation's work. The film also included comments from Meese, budget director David A. Stockman and other administration officials.

"There was no question the film would be used for our own internal fund-raising," Berkowitz said. "It was understood. Neither the president nor any other public officials who appear in the film call on any person to support the Heritage Foundation. They just say what they think of the Heritage Foundation."

Berkowitz said about 1,000 people responded to the mailing, but he could not estimate how much money was raised.

Last January, the foundation told 5,000 potential donors that they could meet with top White House officials by contributing at least $1,000 to join the "President's Club" at the foundation. Enclosing an endorsement from Meese, Feulner wrote that "you will be provided with an access to Washington policy makers which cannot be had at any price."

Meese has addressed two Heritage gatherings, and Berkowitz said the president plans to speak to a conference being sponsored by the foundation this fall. The foundation, which is spending $7.2 million this year on studies and publications, raises nearly half its income through direct-mail appeals and has 140,000 active contributors.