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Transcranial magnetic stimulation of the motor cortex was performed in 10 normal subjects and 10 patients with radiographical abnormalies of the corpus callosum. Seven patients had a complete or partial agenesis or hypoplasia of the corpus callosum, two had a thin corpus callosum due to hydrocephalus or white matter degeneration and one had a circumscript contusion lesion of the corpus callosum. The patients served as a clinical model to investigate transcallosal influences on excitatory and inhibitory effects of motor cortex stimulation and to assess the potential diagnostic use of interhemispheric conduction studies and the contribution of interhemispheric interaction on transcranially elicited contralateral excitatory and inhibitory motor responses. Stimulation over one motor cortex suppressed tonic voluntary electromyographic activity in ipsilateral hand muscles in all subjects with preserved anterior half of …