The structure of neurons and axons in layer IV was studied as part of a larger inquiry into the organization of primary auditory cortex (AI) in the cat. Tissue from the convexity of the middle ectosylvian gyrus between the anterior and posterior ectosylvian sulci was studied in Golgi, Nissl, Bodian, plastic-embedded, and other preparations from adult animals. Layer IV is defined as a strip about 200-250 micron thick consisting predominantly of small non-pyramidal neurons intercalated between the pyramidal somata of layers III and V, and in which few commissurally projecting cells occur. Lying some 800-900 micron beneath the pia, layer IV has six types of neurons, as seen in Golgi-stained material from anatomically and physiologically defined AI. These include three varieties (small, medium-sized, and large) of tufted neurons with intracortically branching axons and vertically polarized, cylindrical dendritic fields. Besides the tufted cells, which are the most numerous neurons in layer IV, large multipolar, double bouquet, and spiny stellate cells are scattered through layer IV. Each has a characteristic neuronal architecture and intracortical axonal branches. Smaller tufted cell somata dominate the upper half of layer IV (IVa), larger tufted cells are more common deep in layer IV (IVb). The average somatic area in midnuclear , plastic-embedded sections is 158 micron2. Layer IV (and layer IIIb) receive thick, probably ascending fibers, forming narrow, vertical terminal fields. These axons may be of thalamic origin and overlap with alternating, 50-75-micron-wide columns of somata and neuropil in layer IV. Layer III pyramidal cell axons often project to layer IV and ramify vertically and horizontally. The average height-width ratio of the dendritic domains of layer IV cells is about 3.6:1. The vertically disposed dendrites of layer IV cells, the columnar arrangement of their local axonal branches, and the polarized form of intrinsic and extrinsic axons collectively reinforces the columnar pattern in layer IV. Many layer IV cells structurally resemble neurons in layer IV in the primary visual and somatic sensory cortex. However, most AI cells have a pronounced columnar arrangement of their somata and an elongated, narrow form. The axons of many layer IV cells preserve this vertical arrangement and often branch in layer III and, to a lesser degree, in layer V.(ABSTRACT TRUNCATED AT 400 WORDS)