The activity of the two folate dependent enzymes supplying carbon 2 and carbon 8 of the purine nucleus was assayed in the bone marrow of rats and man, as well as in rat livers. The activity of both enzymes was several fold greater in marrow than in liver. Inactivation of cobalamin by exposure to nitrous oxide did not affect the enzymes in rat marrow cells, although an appreciable effect on hepatic enzymes was found. The depression of hepatic glycinamide ribotide (GAR) transformylase in rats exposed to nitrous oxide was prevented by supplying a formate precursor, methylthioadenosine. There was a considerable rise in the activity of GAR transformylase in human marrow cells from patients with megaloblastic anaemia due to cobalamin deficiency but no change in activity in marrow from patients deficient in folate.