AbstractMethionine provided in the drinking water of pregnant rats injected with sodium valproate reduced the frequency of resorptions but did not improve embryo growth. Rats drinking methionine supplemented water had approximately twice the level of serum‐free methionine and consumed only one‐half the volume of water of controls. Using whole rat embryo cultures, the simultaneous addition of methionine and sodium valproate to the medium provided no protection from neural tube defects, nor did the addition of methionine to a medium of serum obtained from rats previously dosed with sodium valproate. However, protection from the teratogenic effects of sodium valproate was afforded by methionine when the culture medium was sera from rats consuming methionine and was particularly striking when embryos for culture were taken from pregnant rats that had been consuming methionine. These observations along with those of others indicated the importance of dietary and culture media methionine levels in evaluating experimental and regulatory teratology studies and suggested the possibility that methionine may play an important role in human teratology where multifactorial causes have been implicated in problems such as neural tube closure defects. © 1992 Wiley‐Lis