In rats anaesthetized with urethane, extracellular unit activity has been recorded from neurones in the central nervous system during noxious stimulation of the tail. Accurately graded and sustained stimulation was achieved by immersing the whole tail in water at controlled temperatures. Neurones were found chiefly in the marginal layers of the dorsal horn near the entry of the dorsal roots supplying the tail and in the ventrobasal nucleus of the thalamus; a few neurones were also found in the somato-sensory cortex. Both dorsal horn units and thalamic units showed very similar responses as the tail temperature was gradually raised. At 42 degrees C there was an increase in firing rate which rose sharply with increasing temperatures to reach a maximum at 46 degrees C. At higher temperatures activity declined and at temperatures above 50 degrees C was largely extinguished. The temperature-response curves were bell-shaped. The decline in activity depended on temperature and not on time: sustained firing for many minutes was seen when temperature was at or just below the peak of the bell-shaped curve. The dorsal horn and thalamic cells also responded to noxious mechanical stimulation of the tail. The receptive fields at both levels were similar, being variable in size, often bilateral and sometimes covering the whole tail. None of the central neurones showed any response to noxious stimulation other than on the tail; neither did they respond to movement of the tail nor to light mechanical stimuli applied to the tail or elsewhere. In behavioural experiments conscious rats had their tails exposed to water at various temperatures. The rats lifted their tails from the water at a threshold temperature of 43.7 $\pm $ 0.6 degrees C, i.e. just above the threshold for the central nociceptive neurones. The findings are compatible with a specific nociceptive pathway ascending to the ventrobasal thalamus.