During the last 20 years the anatomical plasticity of the brain in response to sensory stimulation has been clearly demonstrated. This paper reviews the effects of environments rich in sensory stimulation versus those which are stimulus poor. Effects have been noted at all levels from the gross anatomical to the electromicroscopic. As compared to their stimulus-deprived counterparts, animals reared in complex environments tend to display greater cerebral weight and length and cortical depth. The greatest effects occur in the occipital cortex where histological studies have revealed expanded neuron perikaryonal and nuclear size and dendritic branching, more dendritic spines, alterations in synaptic numbers and morphology, and greater numbers of neuroglia. Different cortical areas and layers respond to different degrees. Some responses have also been noted in the underlying hippocampus, particularly in the dentate gyrus. The emerging data present a picture of a dynamic, plastic brain adapting homeostatically to the demands of its environment.