Recordings were made from convergent neurons in trigeminal nucleus caudalis of the rat. These neurons were activated by both innocuous and noxious mechanical stimuli applied to their excitatory receptive fields located on the ipsilateral part of the muzzle. 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 (mean latencies 63.6 +/- 5.5 ms). This type of response was inhibited by applying noxious conditioning stimuli to heterotopic body areas, namely immersing either the left or right hindpaw in a 52 degrees C water bath. A virtually total block of the response was observed 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) (39, 40). The effects on these inhibitions of various transverse lesions of the cervical spinal cord were investigated in acute experiments; tests were performed before and at least 30 min after the spinal section. While the unconditioned C-fiber responses were unaltered, the inhibitory processes could be impaired by the cervical lesions, although these effects depended on the part of the cervical cord destroyed and the side of application of the conditioning stimulus. Lesioning dorsal, dorsolateral, and ventromedial parts of the cervical cord was found not to affect inhibitory processes triggered from either hindpaw. The overlapping of the regions of these ineffective lesions revealed that two remaining regions were not destroyed, that is, the left and right ventrolateral quadrants. In experiments where the left anterolateral quadrant was affected by the surgical procedure the inhibition triggered from the right hindpaw was strongly reduced, whereas that elicited by left hindpaw stimulation was not diminished. The loss of inhibitory effects was characterized by a complete disappearance of poststimulus effects, whereas inhibition observed during the application of the noxious thermal conditioning stimulus was only partially, albeit very significantly, blocked. To ascertain further the mainly crossed nature of the pathways responsible for the heterotopic inhibitory processes, the effects of lumbar commissurotomy were investigated. Again the unconditioned C-fiber responses were unaltered by this procedure, whereas the inhibitory processes, whether triggered from the left or right hindpaw, were strongly depressed in all the experiments.(ABSTRACT TRUNCATED AT 400 WORDS)