Studies of the striate cortex of the kitten are reviewed with reference to the effects of very brief periods of uniocular occlusion. It is shown that changes occur in binocular connections within 4 hours after the vision of an eye is blocked. Effects are also found if the occlusion is instituted while an animal is anaesthetized but not if it is paralysed in addition. If 4-hour unilateral 'patching' sessions are given daily, a cumulative uniocular deprivation effect is found if the animals are kept in darkness between the patching sessions. However, the visual cortex is nearly normal if the kittens are allowed binocular vision between patching periods.