We studied the effects of prenatal protein malnutrition on visual discrimination learning in males of Petaurus breviceps using a Lashley box. Gliders were malnourished or adequately nourished during the prenatal period when their mothers received diets containing low (8%) or adequate (32%, controls) amounts of a protein (casein) throughout pregnancy. The mean weight of adequately nourished neonates was significantly higher (0.47 g ± 0.05 SE) than malnourished neonates (0.31 ± 0.02 g). Animals were first trained to discriminate between black or white stimulus cards affixed to 2 hinged doors. Second, cards containing horizontal or vertical stripes were presented. Finally, cards containing a circle or a square were presented. Gliders received food if they pushed through the door with the correct stimulus card and jumped onto a vertical platform. Malnourished (low-protein) subjects made significantly more errors on all tasks. Analyses of covariance indicated that differences in performance were not a function of body weight.