There is disagreement concerning the role of nitric oxide (NO) in general anaesthesia. The present study was conducted to determine whether the anaesthetic drug isoflurane alters levels of NO metabolites, NOx (NO2 and NO3), in the hippocampus of rats during and after anaesthesia. Results showed resting hippocampal NOx levels of about 20 pmol in freely moving control rats. Five minutes after the induction of isoflurane anaesthesia (4.72%=4 minimum alveolar concentrations, detected by righting reflex), there was loss of the righting reflex coincident with a significant elevation in hippocampal NOx levels. During isoflurane anaesthesia, the maximum NOx concentration rose ≈2.4-fold higher than control levels; the NO3 level increased about 5-fold higher than resting levels. NOx levels returned to control levels following discontinuation of the anaesthetic. When rats were pre-treated with L-NG-nitro arginine methyl ester, an NO synthase-inhibitor, the isoflurane-induced increases in NOx were markedly suppressed. D-NG-nitro arginine methyl ester was ineffective in preventing these neurochemical changes, thus indicating the stereoselective nature of the inhibition by L-NG-nitro arginine methyl ester. Furthermore, L-NG-nitro arginine methyl ester pre-treatment likewise prevented increases in both NO2 and NO3 levels. When rats were exposed to 80% nitrous oxide in oxygen, there was loss of the righting reflex but no change in hippocampal NOx levels. These findings indicate that isoflurane increases production of hippocampal NO and that this may be pertinent to general anaesthetic drug effects.