Nitrous oxide inactivates vitamin B12 with detrimental consequences for folate and methionine metabolism, detectable by an increase in total plasma homocysteine. We hypothesised that a pre-operative vitamin B 12 and folate infusion prevents nitrous oxide-induced homocysteine increase. Sixty-three healthy patients having elective surgery were randomly allocated to receive either B-vitamin plus nitrous oxide; placebo plus nitrous oxide or placebo plus air. Fifty-nine patients completed the study. After intravenous B-vitamin infusion, plasma vitamin B12 and folate concentrations increased 35-fold and 12-fold, respectively, on the first postoperative measurement. Patients who received B-vitamins developed a similar increase (18%) in homocysteine after nitrous oxide (1.9 μmol.l-1; 95% CI 0.2-3.6 μmol.l-1) as those who did not (22%; 2.7 μmol.l-1; 95% CI 0.6-4.8 μmol.l-1). Patients not receiving nitrous oxide had no homocysteine change (0.5 μmol.l-1; 95% CI-0.8-1.9 μmol.l-1), indicating that pre-operative intravenous B-vitamins may not prevent nitrous oxide-induced hyperhomocysteinaemia.