President-elect Ronald Reagan's transition aides denied yesterday that they are intentionaly freezing the U.S. Embassy in Mexico City out of a meeting next month between Reagan and President Jose Lopez Portillo of Mexico.

The embassy was not involved in contacts that led to the visit or in preliminary preparations for it, they said, because Reagan's Jan. 5 talks with the Mexican leader will be unofficial, coming two weeks before the inauguration. Nevertheless, the U.S. ambassador to Mexico, Julian Nava, will be present for the one-day visit in Ciudal Juarez near the border with Texas, they added.

Mexican officials and some U.S. diplomats in Mexico City had reported in conversations with The Washington Post that Nava and his staff were being excluded, embarrassing the Mexicans and irritating the diplomts. Jim Brady, the Reagan transition team's spokesman, said the initial lack of consultation with Nava and the embassy was not intended as a snub. At the same time, he made it clear the transition team felt no obligation to keep the embassy informed.

"The purpose [of the visit] has nothing to do with official State Department business. It's a Reagan trip, a hemispheric gesture of goodwill," he said. "We really didn't request any embassy help. They weren't involved."

Asked whether the embassy was informed of the visit before it was announced in Mexico City, Brady said it was not and added: "We also didn't tell the San Diego Union."

Charles Tyson, the transition team's director of scheduling, said the subject of Nava's attendence came up at a preparatory meeting he held with Mexican officials in Ciudad Jarez on Dec. 19 and 20. At that time, Tyson said he told the Mexicans, the list of those to be invited from the American side had not been decided and so Nava had not yet been invited.

Tyson said he flew from that meeting to Washington and conferred on the subject with Richard Allen, recently named as Reagan's national security adviser. Following their meeting, Allen placed a call to Nava and, once he got the ambassador on the line, invited him to be present.