A colossal stone head, helmeted like a pro football lineman and looking as menacing, now stares over the Mall from the entrance plaza of the Smithsonian's Museum of Natural History.

The Olmec sculpture is the first of nearly 200 treasures of Mexican art and archeology to have their premiere showing in Washington on a three-city tour of the United States. The exhibit, "Treasures of Mexico: From the Mexican National Museums," opens here March 30.

The exhibit spans 12,000 years, through the pre-Columbia epoch to the modern master muralists and painters, including Orozco, Rivera and Siqueiros. It will have dual shows at both the Museum of Natural History and the Hirshhorn Museum and Sculpture Garden.

The 6-foot high stone head, which weighs more than 5 tons, was carefully - very carefully - eased into its outdoor exhibit space yesterday as the first arrival.

The Olmec head, chiseled with stont tools more than 2,700 years ago, is one of the 100 pre-Columbian pieces in the exhibit. It was discovered in 1946 by Matthew W. Stiring, a Smithsonian anthropologist, while on expedition in the Yucatan. The monumental heads, as enigmatic as the Olmec civilization itself, had been toppled or pushed over, some some rolled into ravines.

According to Paul Perrot, Smithsonian assistant secretary for museum programs, it is the largest and most ambitious ever to be sent to the United States from Latin America.

Many of the archeological treasures come from Mexico City's renowned Museo National de Antropologia. At yesterday's press conference, S. Dillon Ripley, Smithsonian secretary, made a rare statement for one museum director talking about another museum.

"It is the most impressive museum in the New World," Ripley said, sprinkling such praise as "most interesting, most dramatic and most stylish."

This will be the first time that some of the objects in the exhibit have left Mexico. Many more have never been seen in the United States.

The oldest object - the carved and incised sacrum (lower backbone) of an extinct ancestor of the camel or Ilama - dates from 10,000 B.C. and is the first known work of art on the American continent. It was found in 1876, lost again, and rediscovered in 1956.

Among the prize Mixtec jewelry in the exhibit is a striking 14-strand necklace of turquoise beads.

The Hirshhorn will mount the exhibit of the works of Los Tres Grandes (The Big Three) as Jose Orozco, Diego Rivera and David Alfaro Siqueiros are known. They were the painters and great muralists of the Mexican revolutionary epoch into the '20s, '30s and '40s.

The dual exhibitions were underwritten by the Armand Hammer Foundation. Hammer, the man who brought the Hermitage art here from Russia, saw an exhibition of Mexican art in Madrid and talked to the Mexican president. Hammer is, as Ripley said in tribute, " a great organizer."