The ultrastructure of feline yolk sacs from 11 stages between the 14th and the 66th day is described with reference to the endoderm and the mesothelium; supplementary histochemical and cytochemical studies are included. Despite the absence of yolk, the endodermal epithelium shows a high degree of differentiation and activity, especially in the period between the 25th and the 38th day. Large stacks of RER, abundant SER, mitochondria enveloped by RER cisternae, and a peculiar type of lysosome are the most prominent organelles. Acid phosphatase, succinic dehydrogenase and NAD- and NADP-diaphorases are found with high activity, whereas the 17beta-hydroxysteroid dehydrogenase assay stains the endothelium only moderately. Indications of reabsorption are less marked. In view of the apparent immaturity of the liver parenchymal cells at this stage, the yolk sac endoderm of cat is suggested to act as an important extraembryonic site of biosynthesis. As preliminary results of a chemical analysis show that the yolk sac fluid has nearly no nutritional value, the substances synthesized are believed to be transported directly to the fetus. The mesothelium shows relatively few alterations over the period studied, is less rich in organelles and is obviously far less active than the endoderm.