Recordings were made from convergent neurons in the lumbar dorsal horn of the spinal cord of the rat. These neurons were activated by both innocuous and noxious mechanical stimuli applied to their excitatory receptive fields located on the extremity of the hindpaw. Transcutaneous application of suprathreshold 2-ms square-wave electrical stimuli to the center of the excitatory field, resulted in responses to C-fiber activation being observed. This type of response was inhibited by applying a noxious thermal conditioning stimulus on the muzzle. The immersion of the muzzle in a 52 degrees C waterbath resulted in a strong reduction of the response during the application of the noxious conditioning stimulus and this was followed by long lasting poststimulus effects. Such inhibitory processes have been termed diffuse noxious inhibitory controls (DNIC). The effects on these inhibitions of lesions including the dorsolateral funiculus (DLF) were investigated in acute experiments: tests were performed before and at least 30 min after the DLF lesion. A lesion including the DLF ipsilateral to the neuron under study completely abolished the inhibitory processes triggered from the muzzle. Concomitantly, a facilitation of C-fiber responses was observed. Nevertheless, DNIC was still impaired even using a juxtathreshold current to elicit a weak C-fiber response. To ascertain further the main, if not entire, participation of the ipsilateral DLF in the descending projections responsible for the heterotopic inhibitory processes, the effects of a lesion of the contralateral DLF were investigated. Neither the inhibitory processes nor the unconditioned C-fiber responses were altered by this procedure. Again, a second lesion including the ipsilateral DLF induced a blockade of DNIC. It is concluded that the descending projections involved in the triggering of DNIC are mainly, if not entirely, confined to the DLF ipsilateral to the neuron under study. The contralateral DLF did not appear to play a role in these processes.