Focal electrical stimulation and microinjection of the excitatory amino acid glutamate in the lateral reticular nucleus (LRN) both inhibit the heat-evoked tail flick (TF) reflex in rats. The stimulation-produced inhibition from the LRN has previously been demonstrated to be mediated by spinal monoaminergic receptors. In the present study, inhibition of responses to noxious thermal stimuli by glutamate microinjected into the LRN was examined and characterized; this study is the first to examine the spinal receptors mediating inhibition produced by selective activation of cell bodies in the LRN. Microinjection of glutamate (100 mM) into the LRN in rats lightly anesthetized with pentobarbital produced a transient (less than 5 min) inhibition of the heat-evoked TF reflex, the magnitude of which increased with the volume of glutamate injected (100, 200, or 400 nl). This glutamate-produced inhibition of the TF reflex was antagonized by the intrathecal administration of phentolamine (30 micrograms), yohimbine (15 and 30 micrograms), or methysergide (15 and 30 micrograms) to the level of the lumbar spinal cord, but was not antagonized by prazosin (30 micrograms) or naloxone (20 micrograms). Yohimbine (15 and 30 micrograms) administered to the level of the cervical spinal enlargement did not significantly alter inhibition of the TF reflex produced by glutamate microinjected into the LRN. Microinjection of glutamate (100 mM, 400 nl) into the LRN elevated TF latencies and hindpaw lick latencies in the hot plate test performed on conscious rats. This inhibition of responses to noxious thermal stimuli in conscious rats was short-lasting (less than 5 min), and was also attenuated by intrathecal administration of yohimbine (30 micrograms) or methysergide (30 micrograms), but not by prazosin (30 micrograms) or naloxone (20 micrograms). While it has previously been established that cell bodies in the LRN mediate descending inhibition of spinal nociceptive reflexes, the present results establish that spinal alpha 2-adrenoceptors and serotonin receptors mediate LRN-produced antinociception and extend our understanding of LRN-mediated modulation of nociceptive responses integrated spinally and supraspinally.