PURPOSE. To better understand the course of cortical maturation during early development, the phenomenon of binocular cross-orientation suppression in neurons of the primary visual cortex (V1) in young infant monkeys was investigated. METHODS. Extracellular single-unit recordings were made in anesthetized and paralyzed monkeys ranging in age between 6 days and 8 weeks. Orthogonally oriented, dichoptic sine-wave gratings were used as visual stimuli. RESULTS. V1 neurons in young infant monkeys showed a higher prevalence and greater magnitude of binocular cross-orientation suppression than in adult monkeys. Binocular suppression decreased and reached an adult level between 4 and 8 weeks of age, the presumed onset-age for stereopsis in monkeys. CONCLUSIONS. During the first 4 weeks of life, the functional connections that are necessary for initiating binocular cross-orientation suppression exist in the monkey primary visual cortex. This finding is consistent with the view that before the abrupt onset of stereopsis, human infants may detect the differences between interocularly iso-oriented gratings and orthogonal gratings.