Analgesia is an important component of general anesthesia. alpha 2-adrenoceptor agonists such as clonidine and dexmedetomidine are effective analgesics at the spinal level, and furthermore, they reduce the volatile anesthetic requirement. In order to probe a possible spinal-level contribution to general anesthetic-induced analgesia, the effects of dexmedetomidine were tested in an isolated spinal cord preparation. The effects of dexmedetomidine were compared with those of isoflurane, and dexmedetomidine-isoflurane interactions were explored. The test response was a nociceptive-related slow ventral root potential (slow VRP) recorded from the isolated neonatal rat spinal cord in response to electrical stimulation of a dorsal root. At 0.2-1.28 vol%, isoflurane reversibly depressed the slow VRP. At a lower concentration (0.14 vol%), isoflurane increased the slow VRP in three of five preparations. At 1.0-1.28 vol%, isoflurane also depressed the monosynaptic reflex. Recovery on washout usually was to a level greater than control. The N-methyl-D-aspartate (NMDA) receptor antagonist (DL)-2-amino 5-phosphonovalerate (10 microM) prevented the rebound to levels above control on isoflurane washout. The earlier components of the slow VRP were more sensitive to isoflurane than were the later. Dexmedetomidine (0.5-10 nM) depressed the slow VRP and had no effect on the monosynaptic reflex. The slow VRP depends on both substance P and glutamate NMDA-receptor-mediated neurotransmission; isoflurance and dexmedetomidine depressed responses to both substance P and NMDA. Although the two agents depress responses to the same neurotransmitters, there is no evidence that they act at the same cellular site(s). There was no significant interaction between dexmedetomidine and isoflurane. The results suggest that isoflurane exerts marked inhibitory effects on spinal neurotransmission, depressing both substance P and glutamate-mediated pathways. There is a possible biphasic effect on the NMDA receptor. To the extent that nociception depends on these neurotransmitters, isoflurane may be expected to exert profound analgesic effects at the spinal level. By blocking responses to strongly arousing stimuli, these effects may contribute to general anesthesia. Suppression of nociceptive neurotransmission at the spinal level may contribute to dexmedetomidine's anesthetic-sparing properties as well as to analgesia by this agent.