To celebrate the country's newest museum complex, the Smithsonian Institution today kicks off 15 days of events marking the opening of the Arthur M. Sackler Gallery and the National Museum of African Art on the Mall.

Informally called the Quadrangle because of the shape of their tract adjacent to the Smithsonian Castle, the two museums open to the public Sept. 28 and form part of a dramatic three-level complex, 96 percent of it underground. Visible at ground level are a 4.2-acre garden named for philanthropist Enid A. Haupt, a circular kiosk and separate entry pavilions for the museums.

The inaugural extravaganza of lectures, symposiums, dinners, concerts, receptions, films and open houses, said Smithsonian Secretary Robert McC. Adams, "marks a new era of international exhibitions ... and the end of 20 years of planning."

Rarely has a cultural institution -- the John F. Kennedy Center for the Performing Arts being a major exception -- opened with such a combination of fancy fanfare and heavyweight lectures. The Smithsonian has issued a cumulative total of 44,000 invitations to its various events.

The quadrangle project cost $73.2 million, including $36.6 million in federal appropriations. It covers 360,000 gross square feet and equals, according to architect Jean Paul Carlhian, the size of three Lincoln Memorials and two Queen Elizabeth 2 cruise ships. The bulk of it lies 57 feet below the ground and has a temporarily closed public tunnel connecting it to the neighboring Freer Gallery and a tunnel connecting it to the Castle for Smithsonian staff. Beneath the kiosk runs a 8,000-square-foot "street," complete with skylights, linking it with an exhibition hall, classrooms and offices.

"The set of complexes ... involve the entire outreach" of the Smithsonian, said Adams, and the round of events is planned to accommodate "... all the constituencies that must be involved and stroked." Tom Freudenheim, assistant secretary for museums, said, "The scale of this thing is immense, and the scale of the openings should follow."

This morning a symposium honoring the late Dr. Sackler, who pledged part of his collection of Asian art to the Smithsonian in 1982, will be held at the National Academy of Sciences. Called "Recreations with the Muses: Creativity and the Human Life Cycle," the panel will discuss "What are the biological, psychological and social sources of creativity?" Scheduled to participate are Frank Press, president of the National Academy of Sciences; Nobel laureates Linus Pauling, Chen Ning Yang and Murray Gell-Mann; scientists Solomon H. Snyder, Donald S. Fredrickson and Jonas Salk; and outgoing Librarian of Congress Daniel J. Boorstin.

In addition, a black-tie dinner and reception will be held tonight inaugurating the Sackler Gallery. Continuing into the weekend are more lectures and receptions and a Kennedy Center concert in Sackler's name.

Starting Tuesday, a round of receptions and lectures will inaugurate the African art museum, which is the heir to the private museum on Capitol Hill founded in 1964 by Warren Robbins. The new building was given to the Smithsonian by Congress in 1979. Adams yesterday called the new museum "the nation's primary study center for African art."

The inaugural symposium next Wednesday at the National Museum of Natural History will discuss "African Art Studies: The State of the Discipline," with four scholars: Adrianus A. Gerbrands of the University of Leiden in the Netherlands, Henry Drewal of the Metropolitan Museum of Art, Rowland Abiodun of the University of Ife in Nigeria and Suzanne Preston Blier of Columbia University.

On Sept. 18 the International Center -- a complex within the complex that consists of an education center, a gallery and a series of offices on the concourse level beneath the kiosk -- will be dedicated and named for S. Dillon Ripley. Ripley, the immediate past secretary of the Smithsonian, was a major force behind creation of the quadrangle complex. His vision, said Adams yesterday, was to establish "an open university."

Also on the schedule are special receptions for the Smithsonian Resident Associates, the Smithsonian regents, the quadrangle donors, members of Congress, the building contractors, minority leaders and teachers.

The Sackler Gallery will house a thousand artifacts from the collection of the renowned medical researcher and art collector who died in May. His donation to the Smithsonian included $4 million toward the construction of the museum. The governments of Japan and Korea each contributed $1 million to the building fund. The building is 110,000 gross square feet with 22,000 square feet of exhibition space. The Sackler is the Smithsonian's 14th museum.

The African art museum has 6,000 objects in its permanent collection and covers 108,000 gross square feet. It has 25,000 square feet of public space and five times more exhibition space than the old location. Included in the new building is a library of 13,000 volumes on African art, history and culture and the expansive Eliot Elisofon Photographic Archives.

A preview of how detailed the opening weeks will be came yesterday as the Smithsonian organized a 90-minute press conference and slide show and opened the galleries for the entire day. From Adams to Ripley to architect Carlhian there was an obvious sense of relief and pride that the complex had been completed.

"The project will become a landmark itself," said Adams, though he said its high-profile construction site had been referred to as everything from a "swimming pool" to "Atomic Waste Site No. 5." The new garden, he noted, was the grazing area of Smithsonian buffalo 100 years ago. When the National Zoological Park was established in 1890, the animals were moved.

Ripley, who has the title secretary emeritus, said the new quadrangle complex "in a sense ... is an answer to a prayer ... It is fascinating, somewhat controversial but successful." One of the controversial points has been the placement of African and Asian art underground. It was a fact that worried architect Carlhian at first. "You never go down to anything important ... my preoccupation from the beginning was to make the descent as exciting as possible."