Contrast sensitivity functions have been obtained in four animals for gratings in vertical, oblique, and horizontal orientations. The data demonstrate that the monkey visual system, like that of humans, shows variations of contrast sensitivity with grating orientation at high spatial frequencies. One monkey showed a classical oblique effect, i.e. similar sensitivity for vertical and horizontal and a lower sensitivity for oblique gratings. The other three monkeys showed contrast sensitivity differences between horizontal and vertical gratings. The similarity of monkey and human contrast sensitivity variations and the implications of these results for use of the macaque monkey as an animal model for human vision are discussed.