Summary Folate coenzymes were determined in the chick embryo, in the nonembryonic portions of the egg and in the whole egg at different stages of development by means of differential microbiological assays. Moreover, the enzymic activities involved in the metabolism of these structures were evaluated in the chick embryo. During development the total folate content of the whole egg remains constant, only in the latest stages does a slight decrease occur. In the embryo, folate activity increases constantly during development, while it decreases in the nonembryonic portions of the egg, in particular from day 12 onward. Similar behavior was observed for each folate derivative. Nevertheless, it is possible to observe in the whole egg as well as in the embryo, significant percentage variations among the coenzymes at various stages of development; particularly on day 12 the amount of H4folate is higher, while that of CH3-H4folate is lower. As for the enzymes studied, an increase of activity between days 10 and 12 was observed. Therefore it is possible that an increase of the synthesis, followed by an increased storage of coenzymes occurs when the demand for the latter increases.