To provide a neurochemical basis for the central nervous system actions of nitrous oxide, the changes of brain dopamine (DA), serotonin (5-HT), norepinephrine (NE), and metabolites of DA and 5-HT were studied in rats. Thirty male Wistar rats were assigned to one of five groups according to the type of gas and the duration of gas exposure. The rats in one group, which served as control, were exposed to air for 30 min, and the rats in four other groups were exposed to 75% nitrous oxide in oxygen for 0.5, 1, 2, and 4 h, respectively. Animals were killed with microwave irradiation, and the brains were divided into seven sections: the cerebral cortex, cerebellum, striatum, hippocampus, midbrain-thalamus, hypothalamus, and medulla-pons. The contents of NE, DA, 3,4-dihydroxyphenyl-alanine (DOPAC), homovanillic acid (HVA), 5-HT, and 5-hydroxyindoleacetic acid (5-HIAA) in each discrete area were measured with high-performance liquid chromatography. Nitrous oxide had no significant effect on the contents of NE, 5-HT, nor 5-HIAA, but decreased that of DA in the striatum and midbrain-thalamus after 4 h of exposure (P < 0.05). Levels of DOPAC, but not DA, in the cerebral cortex and medulla-pons were increased significantly at exposures up to 2 h (P < 0.05), but were not significant from control levels after a 4-h exposure. Increased levels of DOPAC indicate that nitrous oxide increases dopaminergic neuronal activities in the mesocortical projection and the medullary network.(ABSTRACT TRUNCATED AT 250 WORDS)