According to previous evidence, the transfer of the ascending messages through the nucleus veniralis posterolateralis (VPL) of one side is reduced by conditioning volleys produced by direct stimulation of the contralateral VPL. The conditions of the experiments and the time-course of the interaction were such as to suggest the existence of some, «transversal inhibition» between the VPL nuclei of the two sides. The present experiments are intended to assess whether the inhibitory effect is presynaptic or postsynaptic in nature. To this end, direct single-shock stimulation of the VPL impinged upon by the transversal volleys was employed, in order to obtain the typical orthodromic potentials from the radiation, on the one hand, and the antidromic potentials from the afferent prenuclear pathways, on the other. After conditioning stimulation of the contralateral VPL, of the two components characterizing the orthodromic response only the second one (ß component) underwent reduction, thus suggesting the existence of inhibition at presynaptic level. Accordingly, the antidromic potentials exhibited clear increase in amplitude, as is to be expected when presynaptic terminals are depolarized by presynaptic inhibition. Both lines of evidence therefore converge in pointing at presynaptic mechanisms for the «transversal inhibition» between the VPL nuclei.