The cultured rat embryo undergoing organogenesis (9.5-11.5 days of gestation) together with its associated yolk sac synthesize purine nucleotides via the de novo synthetic pathway. Although both the embryo and its yolk sac contain significant levels of the purine base salvage enzymes adenine phosphoribosyltransferase and hypoxanthine phosphoribosyltransferase, the culture medium that consists largely of rat serum contains no measurable quantities of salvageable purine bases or nucleosides but high activity levels of purine catabolic enzymes. Short-term pulse-chase experiments with adenine and guanine, carried out under virtually serum-free conditions, confirmed that purine base salvage mechanisms were active and that there was no significant net transfer of purines between the embryo and its yolk sac. A comparison between the specific radioactivities of the [14C]glycine added to the culture medium for the studies of the de novo synthetic pathway and the purine bases in both the cellular nucleotides and the nucleic acids indicated the existence of a large glycine pool, which almost certainly was derived from the degradation of medium serum proteins by the yolk sac. Although there are no clear-cut data available on the in vivo plasma levels of purines that could be potentially utilized to meet the demands of the embryo, it is evident that the de novo pathway is adequately developed to meet these needs.