In anaesthetized rats, single fibres have been dissected from the tail nerves. Fibres were found which became excited when the temperature of water surrounding the tail was raised above 40 degrees C. Firing rate increased with stepwise increases in temperature, showing first a transient outburst followed by adaptation to a static level. Corresponding neurones were also found in the dorsal horn at the entry zone of the roots coming from the tail. The cord neurones had a higher threshold temperature of 42.5--45 degrees C. When the spinal cord was reversibly blocked by cooling in the thoracic region, then the threshold of the dorsal horn neurones was reduced to that of the afferent fibres. In addition, at suprathreshold temperatures dorsal horn activity was greater during cord blockade. We conclude that dorsal horn neurones responding to noxious heating are subject to a tonic descending inhibitory control.