A series of experiments examined the potential plasticity of the callosal system in both epileptic patients and in kittens submitted to corpus callosotomy at various ages. The patients were tested for unilateral discrimination and interhemispheric transfer of tactile information. The youngest patient was also required to perform additional inter- and intrahemispheric comparisons of visual and tactile stimuli. The animals were tested for interhemispheric transfer of visual discriminations. The results suggest that in both animals and humans there exists a critical period before which callosal section does not disrupt interhemispheric communication. The results also indicate that the compensatory mechanisms used to achieve interhemispheric transfer in the absence of the corpus callosum may vary according to the sensory modality involved. The possible physiological and/or functional mechanisms responsible for callosal plasticity are discussed.