By means of extracellular microelectrodes, 364 ventrobasal units (n. ventrails posterolateralis, VPL) of curarized, unanesthetized cats have been sampled and tested for their reactivity to lemniscal stimulation (medial lemniscus; concentric electrodes; stereotaxic plane A5), to the stimulation of contralateral homonymous nucleus (concentric electrodes; stereotaxic planes A9-A10), and also (in 324 units of the sample) for their reactivity to somatic stimuli in order to ascertain the extent and location of peripheral receptive fields. The experiments were performed in tact or acute decorticate animals, and in cats with chronic lesions of the dorsal or lateral columns of the spinal cord. In neuraxially intactanimals, in acute decorticate preparations, and in cats with lateral spinal lesions, practically all units identified within VPL (98% ) were unaffected by the stimulation of contralateral nucleus and exhibited clear-cut lemniscal properties as to their peripheral fields. In cats with dorsal spinal lesions, on the other hand, a substantial proportion of the units sampled did react to contralateral nucleus stimulation and were activated upon stimulation of widespread peripheral fields. Both of these properties were often found to be associated in the same units. Histological controls showed that such units were preferentially distributed to the caudalmost third of VPL, i.e., where the spinothalamic contribution to this nucleus is known to converge. This evidence is not consistent with the generally-accepted views on the functional homogeny of the whole ventrobasal population. The results obtained are discussed considering this fact and the fact that the critical lesion, and not the other ones, must impair afferent inhibition normally exerted through lemniscal channels. According to previous hypotheses, the existence is assumed of some lemniscal inhibitory drive normally acting against the extralemniscal properties of spinothalamic ventrobasal component. Reactivity to contralateral nucleus stimulation is envisaged as an extralemniscal property itself.