In a simultaneous forced-choice procedure, three pigeons learned to discriminate between a blank field and sinewave gratings of equal space-averaged luminance. Conditions included two spatial frequencies and, for each, a series of contrasts, which were varied over a range sufficient to generate psychometric functions. Drug tests used two doses of morphine and a condition that combined naloxone with the higher dose. Morphine decreased contrast sensitivity, although this outcome was reliable only at the higher dose and the higher spatial frequency. This effect was reduced and usually blocked by naloxone. Morphine-induced changes in pupil size were too small to account for the opiate effect.