"Butch" and "Sundance," two robbers who have stayed at least a step ahead of the law for four months while holding up a string of 11 Northern Virginia pharmacies, made it an even dozen yesterday morning.

The pair, who police said were armed, robbed the Apothecary of Burke, 8996 Burke Lake Rd., south of Fairfax City, at 10:45 a.m. They got an undetermined amount of cash. As in earlier holdups, they also took morphine-based drugs which they demanded by specific brand names.

There have been no injuries in the 12 holdups.

Fairfax County police yesterday issued a composite drawing of one suspect based on witnesses' descriptions. But they also sounded a note of frustration at the pair's repeated successes.

"With all the publicity this has gotten and the fact that [the suspects] apparently use the same method of operation, you would think that some of the people might be on the lookout for these two," said police spokesman Warren Carmichael.

The two men, nicknamed "Butch" and "Sundance" by one of their earlier victims, are referred to as "Salt" and "Pepper" by police because one is white and the other black, according to witnesses' accounts.

The pair are thought to be responsible for six other holdups in Fairfax County, two in Arlington, two in Alexandria and one in Prince William County. CAPTION: Illustration, Composite of robbery suspect.