&NA;Methionine synthetase (MS) activity in the brain and liver is decreased following nitrous oxide (N2O) exposure. Since MS is important for DNA synthesis, this interaction would be expected to have the most serious consequences on actively replicating tissue. The authors therefore measured MS activity in rat testes following 1 h exposure to either 10% or 50% N2O. Animals exposed to 10% N2O had a 29% reduction in MS activity, and exposure to 50% N2O caused a 63% reduction in enzyme activity. Testicular MS activity returned to normal by 24‐48 h in the 10% exposure group and by 72 h in the 50% exposure group. This biochemical effect on testicular enzyme activity could be the basis for the reported deleterious effects of N2O on spermatogenesis.