This is a systematic study of the electrocortical responses evoked by focal stimulation of the magnocellular portion of the medial geniculate body (GMmc). The experiments have been performed on unanaesthetized cats (acute cerveau isolé and chronic, neuraxially-intact preparation). The results obtained show that the interrelationships between GMmc and the cerebral cortex are complex. The projections that have been identified concern several primary and associative cortical areas. In order of decreasing projection density, they can be ranged as follows: insulo-temporal, second acoustic, second somatic, first acoustic, anterior middle suprasylvian associative, first somatic and posterior middle suprasylvian associative areas. In the areas implied, the projections terminate in a diffuse mode and notin several discrete foci. Judging from latency values (1.5-6 msec), the responses appear to be mediated by several different projection systems both direct and indirect, and to utilize,to a lesser extent, also subcortical pathways projecting to the contralateral hemisphere.