Using a sensitive immunohistochemical procedure, we have undertaken a detailed morphological characterization of the vasoactive intestinal peptide-positive (VIP-positive) neuron in the cerebral cortex of the rat. VIP-positive neurons are present in all regions of cortex, and are usually strongly bipolar, possessing long, radially directed processes with very limited branching in the tangential plane. The most extensive dendritic branching occurs in layers I and deep IV-superficial V, and the density of axonal varicosities is highest in layers II-IV. In the visual cortex, approximately 50% of the labeled cell bodies are in layer II and III and 80% of the labeled cell bodies are contained in layers I-IV (superficial 600 microns of cortex). In order to determine the density and 3-dimensional distribution pattern of these cells, we prepared serial tangential sections through the rat visual cortex, mapped the distribution of all labeled cells in each section on transparent acetate sheets, and compressed these superimposed maps. This analysis demonstrated that: (1) approximately 1% of the cortical neurons are VIP-positive, (2) their distribution is fairly uniform and statistically random, (3) there are no large areas (i.e. with a diameter greater than 100 microns) that lack a VIP-positive cell, (4) on the average, there is one VIP-positive cell per column of 30 microns diameter, and (5) the average nearest neighbor distance on the compressed display is 15 microns. Given the morphological characteristics of VIP-positive cells, these data indicate that each VIP-containing cell is identified with a unique radial volume, which is generally between 15 and 60 microns in diameter, and overlaps with the contiguous domains of neighboring VIP-positive cells. These morphological data support the notion that VIP-containing neurons play an important functional role within radially oriented columns of cerebral cortex.