Hepatic methionine synthase activity has been determined in rats continuously exposed (24 h day-1 and 7 day week-1) to concentrations of nitrous oxide ranging from 500 to 50 000 p.p.m. for periods ranging from 24 h to 28 days. The inactivation of this enzyme was dependent upon both time and concentration of nitrous oxide exposure, but there was no statistical evidence that exposure continuing beyond 48 h increased the effect. However, the dose-response curve for 24 h was significantly different from the curve for pooled data from exposures lasting 2-28 days. These latter data indicated no significant effect with 450 p.p.m. and an ED50 of 5400 p.p.m. Significant inhibition was detected at 1000 p.p.m. These results suggest that the limit of exposure of 25 p.p.m. recommended by the American National Institute of Occupational Safety & Health may be unduly restrictive.