In order to investigate the effects of spatial summation on the spinal transmission of nociceptive information, we compared in intact and spinal anaesthetized rats, responses of lumbar convergent neurones elicited by noxious heat stimuli applied to areas of the body much greater in size than their individual excitatory receptive fields, located distally on the hindpaw. Twenty-four neurones were recorded in each group of animals. For each neurone, 4 successive immersions of increasing areas (1.9-18 cm2) of the ipsilateral hindpaw in a 48 degrees C water bath (15-sec duration) were performed with 10-min intervals in a randomized and balanced order. In intact animals, the responses of convergent neurones progressively decreased when the area of noxious thermal stimulation reached and then exceeded approximately twice the area of their individual excitatory receptive fields. This decrease was highly significant for 18 cm2 which represents approximately 10-fold the mean of the receptive field areas. Such a phenomenon was not observed for neurones recorded in spinal animals although their excitatory receptive field areas were not significantly different. These results suggest that the activation of a large population of nociceptive afferents triggers supraspinally mediated negative feed-back loop modulating the responses of convergent neurones.