The excretion of methylmalonic acid was measured and deoxyuridine suppression test (dU-test) was performed in 5 patients before and after 24 h of ventilation with nitrous oxide. The mean urinary 2-methylmalonic acid was increased from 90 to 320 mumol/24 h and the dU-tests became abnormal. These findings indicate that the 2 vitamin B-12 dependent enzymes, methylmalonyl-CoA-mutase and methione synthetase, are depressed by nitrous oxide.