Fasting rats were exposed to nitrous oxide (70% N2O/30% O2) for 24 h, a period of time sufficient to inactivate methionine synthase (correct trivial name). A spectrum of amino acids was measured in the brain, plasma, and liver. In addition, S-adenosylmethionine was measured in the liver and brain and glutathione was measured in the liver. Nitrous oxide caused no changes in the circulating concentrations of amino acids except for a decrease in tyrosine and alanine. Many amino acids were decreased in the liver, including grycine, tyrosine, phenylalanine, and methionine. In the brain, only methionine and tyrosine were reduced. The administration of methionine did not reverse any of the amino acid changes except to increase methionine concentrations in the plasma and brain. Nitrous oxide reduced S-adenosylmethionine in the brain and liver. Administration of methionine to nitrous oxide-treated rats increased the level of S-adenosylmethionine in the liver and brain by 109% and 38% respectively above the control value. Liver glutathione (reduced) was unaffected by nitrous oxide. These results demonstrate that nitrous oxide can decrease the concentrations of two intermediary metabolites (methionine and S-adenosylmethionine) that are important in the control of methylation reactions and in the maintenance of the active form of tetrahydrofolic acid. Although nitrous oxide potentially is a strong oxidizing agent, there was no evidence of a disturbance in the liver glutathione pool.