The topographic distribution of somesthetic interhemispheric projections was studied in grey squirrels using the Fink-Heimer technique following large aspiration lesions of the corpus callosum. On the day of perfusion, receptive fields were determined for microelectrode recording sites in the first, S I, and second, S II, somatosensory areas of cortex, and small electrolytic lesions were made in order to identify some of these sites in prepared brain sections. The cortex was then separated from the rest of the brain, flattened, and cut parallel, so that with the aid of the reference lesions, the total pattern of degeneration could be related to a surface view of the brain and to previous electrophysiological maps of S I (Sur et al., '78) and S II (Nelson et al., '79). The results show that callosal terminations are unevenly distributed in S I and S II, and suggested that there are several categories of callosal inputs to S I. A major region of terminations is in the architectonically distinct "unresponsive zone" within SI, and perhaps in other similar, but narrower, specialized zones within and bordering S I, as previously described in rats (Ryugo and Killackey, '75; Wise and Jones, '76, '78; Akers and Killackey, '78). Other callosal projections terminate within the responsive regions of S I. These regions include at least some of the representation of the body midline, most clearly the midline of the representations of the upper and lower face, as well as regions unrelated to the midline of the body. Most of all of the S I cortex responsive to stimuli away from the midline on the upper and lower lips, the mystacial vibrissae, and the glabrous forepaw was almost free of direct callosal terminations. Except for a central core region, most of S II appears to receive a moderate distribution of callosal inputs.