Correlative light and electron microscopic immunocytochemical methods were used to analyze the 5-HT innervation of the primary auditory area (AI) of the cat cerebral cortex and to examine the synaptic relationships of 5-HT basket terminations on target neurons in that area. Three morphological types of 5-HT-immunoreactive fibers are present: type I, which is very thin and very finely beaded; type II, which is thin and coarsely beaded; and type III, which has a relatively thick main shaft and very few beads. Type I is the most abundant, type II is relatively less common, and type III is the least abundant type. The 3 types of fibers are present through the thickness of AI and in the subjacent white matter, but the densest plexus is found in layers I-III. One of the most characteristic features of type II fibers is that they commonly form small, dense clusters that resemble baskets apposed to the somata and primary dendrites of unstained neurons. The basket formations are more frequently found in layers I and II, and they vary in complexity. Simultaneous immunostaining for GABA and 5-HT reveals that many 5-HT baskets surround the somata and dendrites of GABA neurons. In 2-microns-thick plastic sections, each basket formation can be seen surrounding 1 or a group of 2 or 3 cells. In the latter case, one cell is much larger and at the electron microscope level is identified as a neuron, while the other cells are neuroglial cells. Reconstructions were made from serial electron micrographs of 135 5-HT-immunoreactive boutons. Of these boutons, 110 belonged to basket formations, 14 to type I axons located in the neuropil, and the remaining 11 to type II fibers located in the white matter. Only 4 of the 135 boutons made conventional synaptic contacts. These were of the asymmetrical type. Most of the boutons made very small, indistinct membrane specializations or none at all. The present results therefore suggest a strong interaction between 5-HT axon terminals and specific GABA neurons, which may be mediated by release sites that are not associated with morphologically distinct synaptic contacts.