The effect of acute phenobarbitone treatment on the distribution of endogenous bound folates in three specific cytosolic folate-binding proteins (FBP-C), has been studied in rat liver. The bound folate amount shows no difference between treated and control rats. As it is well known that FBP's preferentially bind longer-chain polyglutamates, the content of which is markedly lower in phenobarbitone-treated rat liver, it might be suggested that shorter chain folates also bind to FBP's in these animals.