During a critical period in early life, physiological studies reveal that either prolonged or brief periods of monocular occlusion induce similar plastic changes in the primary visual cortex (area 17) of the cat, leading to a nearly complete loss of visual responses from the deprived eye (Hubel and Wiesel , J. Physiol. (London) 160: 106-154). However, the recovery of function is markedly different in the two conditions, being complete and thorough only after a brief period of monocular occlusion (Movshon  J. Physiol. (London) 261: 125-174). In search for anatomical correlates that distinguish between these two experimental conditions, we compared the geniculocortical connectivity in normal kittens with that following brief (4 days and 6-7 days) or prolonged (> 5 weeks) periods of occlusion of vision in one eye. Transneuronal labeling of the geniculocortical pathway revealed changes in both cases, and single afferent arbors reconstructed in serial sections were similarly shrunken after short or long periods of deprivation. However, a significant expansion of the geniculocortical afferents serving the nondeprived eye was evident mainly after prolonged deprivation. These findings provide further evidence for rapid, activity-dependent remodeling of afferents during development.