It has been shown that the responses of VB thalamic neurones to noxious mechanical and thermal stimuli are strongly depressed by low doses of Morphine (0,03; 0,1; 1 mg/kg i.v.) and to a lesser extent by ES 52 (a highly potent Thiorphan derivative) injected at higher doses (5; 10 mg/kg i.v). This last effect was less easily reversed by Naloxone than was the depressive effect of Morphine. Moreover, ES 52 can facilitate activity of some thalamic neurones induced by non-noxious mechanical stimuli.