The free surface of the endodermal epithelial lining of the rat visceral yolk sac was examined by scanning electron microscopy at three stages of pregnancy, viz., 12, 17 and 22 days (birth typically occurring on the 23rd day). Additionally, by ultrasonic vibration of tissues that had been subjected to prolonged osmium fixation, the epithelium was removed and such microdissected membranes similarly were examined. With increasing gestational age the free surface of the epithelium underwent a relative increase in absorptive area by three mechanisms: formation of increasingly complex villous projections of the visceral yolk sac as a whole, a doming of the individual epithelial cells, and an increase in length and number of microvilli for each such cell. The resultant increase in surface area, however, could not be correlated with the onset of receptor mediated endocytosis at the latest stage. The basement membrane underlying the lining epithelium of the yolk sac easily was revealed by ultrasonication of well-fixed tissues; it was most fragile at 12 days at which time the procedure apparently either removed the lamina lucida of the basal lamina, exposing a fibrillar component of the lamina densa, or removed the entire basal laminar component of the basement membrane, exposing an underlying reticular lamina. At 17 days the basement membrane at the cores of denuded villi showed the negative impressions of the epithelial cells that had been removed; and at 22 days, when the membrane is known to be permeable to protein transfer, it was smooth, featureless, and appeared most durable.