The morphology of electrophysiologically identified neurons was examined in the primary auditory cortex (AI) of the cat. After stimulation of the medial geniculate nucleus (MG), second auditory cortex, posterior ectosylvian gyrus, contralateral AI, or corpus callosum, intracellular potentials were recorded from AI neurons, which were then injected intracellularly with horseradish peroxidase and recovered. Layer IV neurons, which receive MG fibers monosynaptically, are spiny and nonspiny stellate cells, small and medium-sized nonspiny tufted cells, and fusiform cells. They send their axons to layer III of the AI. Corticocortical AI neurons are medium-sized pyramidal cells in layer III. They receive axons from layer IV neurons of the AI and send their axons to layers I, II, IV, and V of the AI. Horizontal cells in layer I receive slow-conducting MG fibers monosynaptically, and send their axons to layer II of the AI. Stellate cells and small pyramidal cells in layer II receive afferent inputs polysynaptically from the MG. Layer II pyramidal cells receive afferent inputs from the MG via AI neurons in layers I and III, and send their axons to layers V and VI. The axons of layer II stellate cells were distributed within layer II. Pyramidal cells which send their axons to the MG are located in layers V and VI, distributing their axon collaterals to layers III-VI of the AI.