Stereoscopic tilt can be obtained by presenting vertical grating patterns differing in their spatial frequencies to the two eyes (Blakemore, 1970). Following monocular adaptation, the perceived spatial frequency of a test grating is different in the adapted and unadapted eyes by an amount that would yield stereoscopic tilt if corresponding physical spatial frequencies were presented (Experiment 1). Perceptually unequal (but physically identical) gratings presented to the two eyes do not yield stereoscopic tilt, but tilt is obtained if differences in physical spatial frequency exist (Experiment 2). Changes in perceived spatial frequency for a binocular test grating following monocular adaptation are smaller than those measured in the adapted eye but larger than those in the unadapted eye (Experiment 3). These results suggest that (1) stereopsis is immune to perceptual distortions of the monocular stimuli, and (2) perceived spatial frequency reflects the pooled activity of monocular and binocular neurons. © 1987 Psychonomic Society, Inc.