In the postcentral gyrus of the mature rhesus monkey the distribution of callosal projection neurons is discontinuous. The density of callosal projection neurons, which are mainly located in the supragranular layers, varies both within and across cytoarchitectonic areas (Killackey et al., '83). In the present study, we investigated the ontogeny of corpus callosum projections of the postcentral gyrus in five fetal rhesus monkeys, ranging in age from embryonic day (E) 108 to E 133. Multiple large injections of horseradish peroxidase that involved the underlying white matter were made into the postcentral gyrus of one hemisphere and the distribution of labeled neurons in the ipsilateral thalamus and the other hemisphere was determined. The pattern of thalamic label indicated that the tracer was effectively transported from all portions of the postcentral gyrus. We found that the areal distribution pattern of labeled callosal projection neurons varied at the different fetal ages. At early fetal ages (E 108, E 111, and E 119) callosal projection neurons were continuously distributed throughout the postcentral gyrus. As in the adult animal, the vast majority of labeled callosal projection neurons were found in the supragranular layers, although a few labeled cells were located in the infragranular layers. From the earliest age, there was regional variation in the width of the band of labeled supragranular callosal projection neurons. The difference between the precentral and postcentral gyrus was most obvious, but there was also a difference between anterior and posterior portions of the postcentral gyrus. The first indication of some discontinuity in the distribution of callosal projection neurons was noted at E 126. By E 133, approximately 1 month before birth, the distribution of callosal projection neurons appeared remarkably mature. On E 119 aggregations of anterograde label could be detected in restricted portions of the posterior postcentral gyrus beneath the cortical layers. By E 133 anterograde label was found within the cortical layers (most densely in layer IV) in these regions of the postcentral gyrus. Thus, the emergence of the discrete pattern of callosal projection neurons appears to be temporally correlated with the ingrowth of callosal afferents. On the basis of these observations, as well as those of others (discussed in the text), we propose that the ontogenetic changes in the distribution of callosal projection neurons reflect the unique strategy employed by cortical projection neurons in establishing their patterns of connectivity. It is hypothesized that this strategy may involve multiple processes.