In 16 chloralose-anaesthetized, curarized cats, extracellular recordings (glass micropipettes) were obtained from 197 SI neurones, and each one was tested with appropriate electrical stimulation both of their cutaneous receptive fields and of contralateral SI, SII, and anterior lateral associative (ALA) cortical areas. The study of peripheral reactivity showed that 97 neurones were endowed with very restricted, contralateral fields (somatotopic units, Group I), whereas 66 had wide fields, often bi- or ipsilateral (non-somatotopic units, Group II), and the remaining 34 showed no signs of peripheral reactivity (unreactive units, Group III). The stimulation of contralateral SI cortical area proved to be effective on 51 neurones of the sample; 44 of these (22.3%) exhibited excitatory responses (most after latencies consistent with the hypothesis of callosal transmission), 7 more were inhibited. Six neurones of the excited group were driven also by contralateral SII stimulation (although after long latencies); no neurones driven by SII alone or by ALA were found. By correlating the two sets of responses, the highest transcommissural reactivity was seen to characterize Group II neurones (54.5% excited, 3% inhibited); in this respect Group I ranked far behind (8.2% excited, 5.1% inhibited) and Group III was totally unreactive (no units excited, nor inhibited). The prevalence of Group II neurones in the transcallosally-excited sample might betray hitherto unsuspected mechanisms for the interhemispheric transfer of peripheral messages, including those from distal portions of limbs.