Background A major effect of general anesthesia is lack of response in the presence of a noxious stimulus. Anesthetic depression of spinal sensory neuronal responses to noxious stimuli is likely to contribute to that essential general anesthetic action. The authors tested the hypothesis that gamma-aminobutyric acid receptor type A (GABA(A)) and strychnine-sensitive glycine receptor systems mediate halothane depression of spinal sensory neuronal responses to noxious stimuli. Methods Extracellular activity of single spinal dorsal horn wide dynamic range (WDR) neurons was recorded in decerebrate, spinal cord transected rats. Neuronal responses to noxious (thermal and mechanical) and nonnoxious stimuli were examined in the drug-free state. Subsequently, cumulative doses (0.1-2.0 mg/kg) of bicuculline (GABA(A) antagonist) or strychnine (glycine antagonist) were administered intravenously in the absence or presence of 1 minimum alveolar concentration (MAC) of halothane. Results Halothane, 1.1%, depressed the response of WDR neurons to both forms of noxious stimuli. Antagonists, by themselves, had no effect on noxiously evoked activity. However, bicuculline and strychnine (maximum cumulative dose, 2.0 mg/kg) partially but significantly reversed the halothane depression of noxiously evoked activity. Similar results were seen with most, but not all, forms of nonnoxiously evoked activity. In the absence of halothane, strychnine significantly increased neuronal responses to low threshold receptive field brushing. Conclusion Halothane depression of spinal WDR neuronal responses to noxious and most nonnoxious stimuli is mediated, in part, by GABA(A) and strychnine-sensitive glycine systems. A spinal source of glycine tonically inhibits some forms of low threshold input to WDR neurons.