The present study examined effects of environmental complexity on the size of the superior colliculus, a subcortical structure involved in visuomotor functions. Long-Evans hooded rats raised together in a complex environment for 48 weeks were compared with their littermates housed in individual cages. The depth and area of the superficial gray layer of the superior colliculus were about 5-6% greater in the group from the complex environment, while the deeper layers of the superior colliculus showed no significant differences. The magnitude of the differences approached those reported for the neocortex, which has been considered to be distinctive in its morphological responsiveness to differential environmental complexity. The findings also extend previous observations that visual deprivation leads to shrinkage of the superficial gray layer and indicate that the morphology of this subcortical visual area is responsive to varying degrees of environmental stimulation.