The Samuel F.B. Morse painting "The Gallery of the Louvre," which was purchased in July for $3.25 million, a record price for an American painting, will be shown at the National Gallery of Art at the end of this year or the beginning of next year, its owner, Daniel Terra, announced yesterday.

Terra, who is President Reagan's cultural ambassador at large, made the announcement yesterday at a press conference at the Grey Art Gallery of New York University. The conference marked the opening of the largest Samuel Morse retrospective ever. It includes "The Gallery of the Louvre."

The date of the painting's debut in Washington is still uncertain, according to John Wilmerding, the National Gallery's curator of American art. "We had suggested toward the end of the year," said Wilmerding. "We made the point that a lot of tourists were coming to town. The painting would be a wonderful attraction. Or, if Terra was interested in taking advantage of the opening of the ground floor of the West Building -- several major exhibitions open at that time -- that's the end of January."

Display possibilities for "The Gallery of the Louvre," which depicts 38 paintings hanging in a salon of Paris' Louvre, have not been worked out. "I haven't even done homework on its size," said Wilmerding. "One suggestion would be to hang it in the 19th-century room . . . with our other large American 19th-century works. The other thing would be to use our large courtyard lobby outside the Italian galleries.

"Although it's an American painting, it includes in it all those Italian paintings in the Louvre," Wilmerding said. "It would be interesting to have it near our own Italian paintings. You have the Mona Lisa prominent in it, and then near the painting would be the one da Vinci we have." The National Gallery has the only painting by Leonardo da Vinci in the United States, he said.

Wilmerding said he hopes the museum can show the painting for two or three months. The museum had written to Terra after the purchase was announced. "It's a painting that has seldom been seen by a large public," Wilmerding said. "Obviously Washington would be a good place. He called back and said he would certainly be interested in a Washington opening." The painting goes to Terra's own museum in Evanston, Ill., after the New York show closes in late October. It will also travel to the Louvre in the spring of 1984.