Unilateral stereotaxic lesions were placed in the medial geniculate body (MGB) in 13 cats, and the ensuing axon degeneration was traced to the cerebral cortex by the method of Nauta and Gygax (1954). The anterior and middle portions of the principal part have heavy projections to the middle ectosylvian gyrus (AI), the lateral bank of the middle suprasylvian sulcus (SF), and the most dorsal parts of the sylvian and the anterior ectosylvian gyri (AII) in an orderly manner anteroposteriorly and dorsoventrally. They also send fibers to the dorsal part of the anterior sylvian gyrus, insular area. The posterior portion of the principal part projects fibers massively to the dorsal part of the posterior ectosylvian gyrus (Ep) and the adjoining part of AI. These projections appear to be organized in both anteroposterior and dorsoventral dimensions. The posterior portion gives off a fair number of fibers to limited caudal parts of AII and of the posterior sylvian gyrus. In addition, the principal part of MGB tends to project sporadic fibers diffusely to AI, AII, Ep, SF and the dorsal part of the insulotemporal region. The magnocelJular part of MGB sends marked projections to AII and the adjacent part of AI in a topical manner in the anteroposterior dimension. Its major portion projects fibers to the insular area. The magnocellular part tends to send scattered fibers to the cortex more extensively toward the temporal pole and the sensory areas than does the principal part.