Single neuronal activity was recorded extracellularly from the Forel's field (FH), subthalamic region immediately rostral to the FH (STRF), rostral end of the medial subthalamic region (RE) and lateral hypothalamic area (LHA) of the anesthetized cats. Many of the FH, RE and LHA neurons were excited by nociceptive stimulation such as pinching the skin with serrated forceps and/or intra-arterial injection of bradykinin. These nociceptive neurons were also excited by non-nociceptive stimulation such as tap of deep tissues, bending hairs with an air-puff and/or joint rotation. On the other hand, inhibition by both nociceptive and non-nociceptive stimuli was seen in and around the rostral end of the FH including STRF. 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.