AbstractThe effects of 40, 50, and 60 mg/kg streptozotocin (SZ) on the body weights and the glucose concentration and the osmolarity of the serum of adult rats were determined. Serum prepared from these SZ‐dosed rats was used in embryo culture experiments to investigate effects of diabetic serum on rat embryos during organogenesis. The diabetic serum resulting from each of the tested doses of SZ was teratogenic to 9.5‐day rat explants (embryos and their membranes), causing a range of dysmorphic lesions including cranofacial defects, heart defects, and abnormalities of the branchial arches and the otic capsules.Explants cultured in serum prepared from rats dosed with 60 mg/kg SZ also showed abnormal morphology of both the visceral yolk sac and the embryonic blood cells in the yolk sac capillaries.The development of explants repeatedly transferred between control and diabetic serum indicated that the severity of the dysmorphic effect was dependent on the duration of exposure to diabetic serum. The alternation of sera did not in itself appear to be damaging to the embryos.Explants cultured in control serum, control serum with its glucose concentration increased to that of the diabetic serum, or diabetic serum all took up the same amount of glucose from their culture medium; 30% of the embryos from the diabetic serum were abnormal compared to only 4% from the control serum and the control serum plus gluc