The effects of nitrous oxide on the brain norepinephrine (NE), dopamine (DA), and metabolites of DA were studied in rats. Forty-eight male Wistar rats were divided into 2 groups and exposed to gas in Plexiglas tubes: compressed air or 75% nitrous oxide in oxygen. Each group was further divided into 4 subgroups, consisting of 6 rats each, according to the duration of exposure: 0.5, 1, 2 and 4 hours. The control group of 6 rats were kept in rat cages and exposed to room air. The animals were sacrificed with microwave irradiation after the exposure. Brain contents of NE, DA, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were measured with high-pressure liquid chromatography. Exposure to compressed air in the tube did not affect NE, DA, DOPAC nor HVA. Nitrous oxide did not change the brain content of NE or DA. Metabolites of DA increased with nitrous oxide: DOPAC and HVA increased after 0.5 h exposure (from 105 +/- 8 to 152 +/- 38 ng/g brain, from 120 +/- 9 to 175 +/- 27 ng/g brain, respectively, P < 0.05). Increased levels of DA metabolites indicate that nitrous oxide increases dopamine turnover. However, these increases attenuated during exposure. The attenuation indicates that the decline of DA turnover may correlate with a development of acute tolerance to nitrous oxide action(s) in the central nervous system.