The Smithsonian Institution is giving $1 million presents to four of its museums - the money to be spent on "major acquisitions" - for instance, pictures costing more than $200,000 each.

The Freer Gallery of Art, the Hirshhorn Museum and Sculpture Garden, the National Collection of Fine Arts and the National Portrait Gallery have been told to spend their monies within the next five years. Their funds have not been touched yet, though the National Collection is considering an American 19th-century painting that it may well buy.

Two other Washington museums - Natural History and History and Technology - have been given similar, but smaller, gifts of $500,000 each, and both have started buying.

Because the things that they collect don't cost as much as major paintings, they have been allowed to buy objects, and collections, in the $100,000 range.

The Museum of Natural History, by special dispensation, already has bought two collections - one of 50 million-year-old fossils, one of Mexican masks - that cost $70,000 each. The Museum of History and Technology meanwhile has acquired, for $100,000, a quartet of 18th-century Italian stringed instruments: two violins, a viola and a cello.

The cash comes from the Smithsonian's "unrestricted trust funds," monies from such sources as museum shops, cafeterias, parking garages, and the Smithsonian magazine. In fiscal year 1977, the Smithsonian earned $9.6 million, which $5.5 million went into its endowment. The Congress has told the Smithsonian to save less and spend more. The establishment of the trust fund major acquisition program is one result of that instruction.

Half of the monies going to each museum must be matched by private gifts. If the Hirshborn, for example, raised $500,000, it might then buy a $1.5 million picture. If it didn't raise a cent, it could only spend a third of that amount.

Charles Blitzer, the assistant secretary for history and art, believes that all of the museums will raise the needed funds. "This five-year program is only two months old, and already the museums have come up with $600,000."

If their fund-raising efforts prove successful, the six Washington museums will spend at least $7.5 million on major acquisitions in the next five years.

The Congress does appropriate each year considerable sums for museum acquisitions - the Portrait Gallery gets $300,000; the National Collection, $200,000; the Hirshhorn receives $200,000, and History and Technology will this year be allowed to spend $100,000. But because each museum director must satisfy numerous curators, these federal funds traditionally have been spent on a wide variety of less expensive things.

"And the museums are not allowed to save their appropriated acquisition funds," notes Blitzer. "By law they have to spend them in a given year."

"The museuems of the Smithsonian Institution, in most respects, are very well supported," wrote Ripley in an Aug. 16 memo to their directors. "One major deficiency, however, has been the inadequacy of funds to purchase major items."

Officials at the Museum of Natural History appear to be delighted with their first two acquisitions. The collection of American fossils from the "Lower Eocene Green River Formation" was purchased from a Swiss, H.J. Kirby Siber. It includes 120 fossilized insects, 36 fishes, 110 leaves and flowers (fossilized flowers are rare), a "stingray of truly exceptional quality," a kind of extinct sparrow "better preserved than any found to date," and a "beautifully preserved" bird which "appears to be related to the modern sunbitterns (Eurypygida). If so, it would be the only known fossil for the entire family."

The 180 Mexican masks bought by the same museum were collected by the late Donald Cordry of Texas, a scholar who discussed them in detail in a recent book.

Some show saints, some devils, some armadillos, snakes, tigers, and other beasts. One is solid sivler. "I believe that this collection will be a major acquisition for the Smithsonian," wrote William W. Fitzhugh, chairman of the museum's department of anthropology. "It is published and documented; it can never be duplicated or matched; it has great potential for exhibit . . ."

Marvin Sadik, director of the National Portrait Gallery, has not yet chosen what to buy. "When the great Houdon bust of Benjamin Franklin sold at auction for $300,000 we weren't even close. Now, if something sensational comes along, we have the ammunition to go after it."

Joshua Taylor, director of the National Collection, already is considering a 19th-century American picture, but he says he is proceeding "with considerable caution. Once the merchants discover you have money in your pocket, it is amazing how values change." Taylor says that his museum will acquire both 19th- and 20th-century American works.

Abram Lerner, director of the Hirshhorn, notes that "$200,000 sounds like a lot of money, but it is no longer what it once was. Single 20th-century pictures, major Pollocks, for example, have sold for 10 times that amount. You can't draw up a battle plan for major acquisitions. All you can do is wait, and search, and when a great object comes up - grab it."

The National Gallery of Art is only nominally under the Smithsonian. Though it has often purchased multimillion-dollar pictures, it has never done so with public or Smithsonian funds.