The neuroactive peptides, substance P (SP), methionine-enkephalin (ME), somatostatin (SS) and neurotensin (NT), have been implicated recently in synaptic transmission processes at the level of spinal cord areas intimately involved in sensory integration. The purpose of this project was to examine the possible actions of four of these peptides on the excitability of functionally identified dorsal horn and dorsal spinocerebellar tract (DSCT) neurons in the cat spinal cord in vivo, and dorsal horn neurons in rat spinal cord slices in vitro. Neuronal activity was monitored extracellularly, while the neuropeptides were applied by iontophoresis;SP caused a strong excitation of all tested neurons selectively activated by noxious stimuli, or receiving an input in A(delta) and C primary afferents (PAs), and located in laminae I-III of the cat spinal cord. The majority of units adequately activated by innocuous skin stimuli, however, were not affected by SP. SP also excited a third of the DSCT neurons tested in the intact cat spinal cord, although in most of these units, the excitation was slight when compared to the SP-produced excitation in laminae I-III. ME and SS had selective depressant actions on the excitability of nociceptive cat spinal neurons located in laminae I, II and V. In contrast, the majority of units activated by innocuous skin stimuli were either not affected or weakly excited by ME and SS. Naloxone antagonized the ME depression in all nociceptive neurons, while not affecting responses of units activated by innocuous stimuli, indicating that ME was probably acting upon opiate receptors. SS actions were not modified by naloxone. NT caused a slight to moderate excitation of about 2/3 of all tested cat spinal neurons in laminae I-III. This excitation was not limited to a single population of neurons, however, but was observed in all categories of units recognized in this area on the basis of their excitability by different kinds of cutaneous input. These results in vivo suggested that all four neuropeptides, SP, ME, SS and NT, may play a role in synaptic transmission processes at synapses between PA fibers and dorsal horn neurons in the cat spinal cord. Moreover, it appeared that SP, ME and SS act principally at synapses of spinal nociceptive pathways, while NT did not show such selectivity;In order to determine a more precise site of neuropeptide action, the rat spinal cord slice preparation in vitro was developed. Responses of the dorsal horn neurons in vitro to iontophoretic application of SP, ME and SS were qualitatively similar to those obtained in the intact cat spinal cord. Since SP, ME and SS actions persisted during synaptic transmission blockade with Ca('++)-free, Mg('++)-high bathing media, it appeared that these neuroactive peptides were acting mostly on postsynaptic sites of the dorsal horn neurons tested.