Nitrous oxide, by inactivating cobalamin in vivo, produces a suitable animal model for cobalamin 'deficiency.' The synthesis of folate polyglutamate with tetrahydrofolate as substrate is severely impaired in the N2O-treated rat, but is normal with formyltetrahydrofolate as substrate. Methionine restores the capacity of the N2O-treated rat to utilize tetrahydrofolate the minimum effective dose being 16 mumol. S-Adenosylmethionine was somewhat less effective than methionine but 5'methylthioadenosine, a product of S-adenosylmethionine metabolism, was significantly more effective than methionine in correcting the defect in folate polyglutamate synthesis. 5'Methylthioadenosine is metabolised to yield formate. It is suggested that these compounds have their effect in correcting folate polyglutamate synthesis by supplying formate for the formylation of tetrahydrofolate. Formyltetrahydrofolate, at least in the cobalamin-inactivated animal, is the required substrate for folate polyglutamate synthesis. Cobalamin is concerned with the maintenance of normal levels of methionine and this in turn is a major source of formate through S-adenosylmethionine and 5'methylthioadenosine.