There it was, right at the entrance to the Renwick. The sign -- which read, "We regret that the exhibition, The Art of Russia, 1800-1850, sponsored by the Ministry of Culture of the U.S.S.R. and the Committee International Co-operation has been closed at the request of the Embassy of the U.S.S.R."

The reason for the close of the exhibit at the Smithsonian's Renwick Gallery was the joint recital of Russian mezzo-soprano Renata Babak and baritone Nikita Rosanoff-Wells yesterday afternoon. Babak, now a Washington resident, refected to the West from the Bolshoi Opera in 1973.

The Babak-Rosanoff-Wells recital, "The Art Song of 19-th Century Russia," was to have been part of the Russian art exhibit. According to James Morris, the director of the Smithsonian's Performing Arts Division, the Smithsonian had no idea that Babak was a Soviet defector when the recital was planned, this in spite of the fact that Babak's defection was widely known in music circles.

Reached by phone, Morris noted that the go-ahead for the Babak concert was entirely "a Smithsonian decision," and that the Smithsonian "made no attempt to reschedule it" after learning that Babak had defected six years ago. Asked whether the Smithsonian would have indeed done so had it known about Babak's defection, Morris replied, "That's too conjectural to answer."

Babak herself wouldn't comment on the incident.

The enthusiasm of the capacity audience for this recital may have had more to do with the international problems attendant to it than for its purely musical value.

First, most of the songs and arias sung by Babak and Rosanoff-Wells were by composers hardly household names here: minor figures in Russian music, such as Alexander Dargomyzhsky and Alexander Gretchaninow. There were several selections by Anton Rubenstein, Glinka and Tchaikovsky in addition to a couple of folk songs.

Second, while Babak and Rosanoff-Wells tend to anunciate well, both are singers of considerably less than first-rate ability. Babak, in particular, has an unattractive voice that tends to be very strident and strentorian in the upper register, lacking a true mezzo voice. The instrument lacks subtlety and color and shows that instead of singing in a bel canto manner, Babak prefers the "can belto" style.

Rosanoff-Wells did well in some of the Russian parlor songs he chose, but elsewhere he displayed pitch problems and interpretive difficulties.

Both singers were backed solidly by local pianist Frank Conlon.