In this study we determined the effect of nitrous oxide (N 2 O) on homocysteine concentrations in children and whether blood levels of folate, vitamin B12, B6, methylmalonic acid (MMA) and methylenetetrahydrofolate reductase C6777T genotype affected this relationship. Homocysteine was measured before and after N 2 O. Vitamin levels, MMA and genotype were determined preoperatively. Median age of the 32 participants was 11 months (3-126 months). All children had folate and B6 levels above deficiency values (7.4, 20nmol/L respectively). Five children had MMA levels indicating deficiency (≥0.21µmol/L). Postexposure homocysteine concentrations increased by 25% (P= <0.001). Duration of exposure and initial homocysteine concentrations were predictors of the increase (r 2 = 0.821, P= <0.001).Vitamin B12 and initial homocysteine concentrations were inversely associated (r 2 = 0.277, P= 0.004). Folate, vitamin B6 and genotype showed no effect. In conclusion, N 2 O exposure leads to increased homocysteine in children. Studies investigating benefit of pre-surgical vitamin B12 supplementation may prove worthwhile.iii ACKNOWLEDGMENTS