The yolk-sac function and development of rat embryos grown in vitro for 24hr, starting on day 10.5, were compared with those of embryos grown in utero. The embryos grown in vitro had significantly fewer somites, shorter crown-rump length and smaller yolk-sac diameter when compared with the embryos grown in vivo but all values were within the normal range for this stage of gestation. Head length was not significantly different between the two groups. The cellular and nuclear volumes (Coulter counter) of nucleated yolk-sac red blood cells did not differ significantly between the two groups. RBC cell-cycle analyses by flow cytometry did not reveal any difference between in vitro and in vivo embryos. The clinical chemistries of embryo-yolk-sac homogenates were compared. Protein, triglyceride, lactate dehydrogenase, cholesterol, urea nitrogen and glutamic-oxalacetic transaminase concentrations did not differ significantly between the two groups. These data indicate that impaired yolk-sac function could, in part, be responsible for the developmental delays and the short survival times of cultured embryos.