Single neuronal activity has been recorded extracellularly from the nucleus centralis lateralis (CL), ventralis lateralis (VL) and medialis dorsalis (MD) of the cat thalamus. The majority of the CL, VL and MD neurons were excited by nociceptive stimulation such as pinching the skin with serrated forceps and/or intra-arterial injection of bradykinin. The nociceptive neurons were also driven by non-nociceptive stimulation such as tap of deep tissues, bending hairs and an air-puff and/or joint rotation, and their receptive fields were large. After intravenous administration of either morphine or pentazocine, most nociceptive neurons became unresponsive to nociceptive stimuli, although they were driven by non-nociceptive stimuli. This suggests that morphine and pentazocine have a specific antinociceptive action on these nociceptive neurons. Intravenous naloxone reversed the antinociceptive action of morphine, but failed to reduce the action of pentazocine. This differentiation has an important functional significance.