A major exhibit of American art, from its beginnings to the present, will be sent to Mexico next year by five Washington museums.

The plan was announced yesterday at a news conference at Blair House with Mexican first lady Carmen Romano de Lopez-Portillo in Washington with her husband on a state visit.

"The people of two countries can best understand one another through the sharing of art and culture," she said.

Livingston Biddle -- chairman of the National Endowment for the Arts, one of three cultural agencies cooperating in the venture -- said the idea "stemmed from a visit I made to Mexico two years ago with Mrs. Carter. We thought it would be useful to have a program of cultural exchange."

Biddle said no specific works or central theme had been selected. The museums helping to make arrangements -- and contributing from their holdings -- are the National Gallery of Art, the National Collection of Fine Arts, the Phillips Collection, the Corcoran Gallery of Art and the Hirshhorn Museum.

Also taking part in the planning are the National Endowment for the Humanities and the International Communications Agency.

The exhibit, Biddle said, was a gesture of appreciation to Mexico for major Mexican exhibits recently shown in the U.S. The show is scheduled to open in Mexico City in the fall of 1980 at the Palace of Fine Arts.