Stereotaxic lesions made in the MGB lead to the finding of Nauta-stained degeneration in the anterior, middle and posterior ectosylvian gyri, i.e., areas AI, AII, Ea, Ep, SF and I-T. Degeneration was not found in SII except in 2 brains in which the ventrobasal complex was probably involved in the lesion. There was no significant projection from the MGB to the marginal gyrus, suprasylvian gyrus or precuciate gyrus. Small lesions in MGB produced continous distributions of cortical degeneration, and no evidence was found for multiple discrete projections. There was some indication that the more rostral parts of MGB project more rostrally on the ectosylvian gyrus. The more medial lesions in the MGB affecting the magnocellular nucleus caused a spread of degeneration further towards the temporal pole and insulotemporal area than the more lateral lesions.