A single unit analysis study of the claustrum to somatic electrical stimulation and to some activation modalities has been performed on unanaesthetized curarized cats. Out of 72 units identified in the claustrum, 32 were not activated by somatic stimulation, 40 were securely responsive, but did not all show the same functional properties. Their greatest number (72.5%) exhibited widespread ipsi- and contralateral receptive fields, but the remainder (27.5%) were related to rather restricted somatic areas, generally circumscribed to proximal segments of contralateral limbs. According to their shorter (7-23 msec) or longer (exceeding 40 msec) latency values, the responses obtained can be divided in two groups. Claustral units reactive to somatic stimuli did not all show aspecific properties of convergence, since to some extent the units related with restricted fields (27.5%) appear to be endowed with specific properties. Destruction or inactivation of the cerebral cortex of the hemisphere ipsilateral to the claustral zone explored did not abolish the responsiveness of claustral neurons to peripheral impulses.