In anesthetized and immobilized Japanese monkeys (Macaca fuscata), intraretinal conduction velocities of the ganglion cell axons were measured. The field potentials elicited by optic chiasm shocks consisted of fast and slow components with estimated conduction velocities of 1.19 and 0.72 m/s in recordings from the optic nerve fiber layer, and 1.65 and 1.00 m/s in recordings from the ganglion cell layer. Single cell recordings verified that the time course of the fast component corresponded to the antidromic spike latencies of Y-like cells, whereas that of the slow component covered the latency range of both X-like and W-like cells. In an electron microscopic study of the cross-sections of the intraretinal optic nerve fiber bundles, the axon diameter histograms of large samples (n = 3000-6000) all showed a unimodal distribution with a sharp peak at 0.3-0.6 micron and a long tail extending to 2-3 micron. The mean diameter was largest in the ventral and nasal bundles, smallest in the papillomacular bundle and intermediate in the dorsal, upper arcuate and lower arcuate bundles. However, diameter histograms of a small number of regional axons (n = 255-300) showed a broad tail distinct from the peak at 0.3-0.6 micron, enabling us to segregate a group of larger axons from the medium-sized to small axons. From such regional axon diameter histograms we estimated the mean relative occurrences of the larger axons (7.1-11.3%) and their mean diameters (0.9-1.3 micron). We further applied this relative frequency to the unimodal distribution of the histograms with larger samples in the upper and lower arcuate bundles and estimated the mean axon diameter of the large axons (1.1 micron) and that of the medium-sized to small axons (slightly below 0.5 micron). Finally, in studying the relation between axon diameter and conduction velocity in the two arcuate fiber bundles, we found it to be somewhat different from that previously reported for the cat retina.