The cytoarchitecture, myeloarchitecture, neuronal architecture, and intrinsic and laminar organization of layer II were studied in the primary auditory cortex (AI) of adult cats. The chief goal was to describe the different types of cells and axons to provide a framework for experimental studies of corticocortical connections or of neurons accumulating putative neurotransmitters. A further goal was to differentiate layer II from layer III. Layer II extends from 150-200 micron to about 400 micron beneath the pia and has two subparts. The superficial stratum, layer IIa, has many small, chiefly non-pyramidal neurons, primarily with round or oval perikarya, and a sparse, fine, and irregularly arranged axonal plexus. Layer IIb somata are larger and more densely packed and there is a more developed vertical and lateral axonal plexus. The border with layer III was marked by numerous large pyramidal cells with a thicker apical dendrite with more developed basal dendritic arbors than those of layer II pyramidal cells. Eight varieties of neurons were recognized in Golgi-impregnated material. These included small and medium-sized pyramidal cells, whose apical dendrites often ramified in layer I; bipolar and bitufted cells with polarized, sparse dendritic arbors; small smooth or sparsely spinous multipolar cells with radiating dendrites and small dendritic fields; spinous multipolar cells, whose large dendritic fields had more extensive apical than basal arbors; large sparsely spinous multipolar cells with smooth, robust apical dendrites; tufted multipolar cells with highly developed apical dendrites and some dendritic appendages; and extraverted multipolar cells with a broad, candelabra-shaped dendritic configuration, and with most dendrites oriented at right angles to the pia. The axons of the different cell types had the following general dispositions: those arising from the pyramidal cells could often be traced into the white matter but had many local branches as well; those of the other neurons had more or less extensive local axonal collateral systems and fewer branches which appeared to be corticofugal. However, the complete trajectory of the axons was not always impregnated in the adult material.(ABSTRACT TRUNCATED AT 400 WORDS)