Intracellular recording and iontophoresis of horseradish peroxidase were used to study the morphology of physiologically characterized W-cells in the cat retina. The recording experiments were performed in an in vivo preparation to allow the responses of these retinal ganglion cells to be compared with previous functional studies of these neurons. The physiological and morphological characteristics of 16 injected and recovered retinal W-cells were compared with similar data from 14 retinal X-cells injected in the same preparations. The soma sizes of retinal W-cells were found to fall into two distinct groups. The somata of the phasic W-cells, at every eccentricity, were smaller than the somata of tonic W-cells, with no overlap between the two distributions. Soma sizes of the tonic W-cells fell into the previously described “medium-sized” range of retinal ganglion cell soma sizes and were similar to, although slightly larger than, the soma sizes of physiologically identified beta- or X-cells. The dendritic arbors of all of the cells physiologically classified as tonic W-cells were similar. Every example of this type had four to five primary dendrites that branched a short distance from the soma to form a circular or cruciate dendritic arbor. The dendritic arrays of these cells were easily distinguishable from the compact dendritic arbors of the physiologically identified X-cells. The dendritic arbors of the phasic W-cells were much more heterogeneous, ranging from sparse, wide dendritic arbors to very compact dendritic arbors with many fine branches. No significant correlation was found between the extent of the dendritic arbor and the distance from the area centralis for either the tonic W-cells or the phasic W-cells. The axons of the tonic and phasic W-cells differed from one another and from X-cells on a number of different morphological and physiological measures. The intraretinal segments of the axons of the phasic W-cells had the smallest diameters of the three groups; the axons of X-cells in the retina were relatively large, and the axons of the tonic W-cells had diameters intermediate between the phasic W-cells and the X-cells. Although considerable overlap was seen between the X-cells, tonic W-cells, and phasic W-cells in their antidromic latencies to electrical stimulation of the optic chiasm, the intraretinal and extraretinal components of the conduction velocities of the three groups were significantly different.(ABSTRACT TRUNCATED AT 400 WORDS)