Pregnancies in women with type-1 diabetes are associated with an increased risk of developmental disorders. Despite extensive research, the toxicological mechanisms and pathogenesis of these disorders are far from explained. The whole rat embryo culture system was used to study the pathogenesis of malformations induced by culture in sera from diabetic women. In undiluted control sera, rat embryos with 4–8 initial somites developed well. During 26 hr of culture, on average 13 somites were formed per embryo, corresponding to the normal in vivo development. Only 3 out of 59 embryos had a significantly lower morphological score than did controls after culture. In the diabetic group no increased incidence of malformations was found, and development tended to be better than in control sera. Embryos cultured in sera derived from conventionally treated patients had a higher morphological score after culture than did embryos cultured in sera derived from women treated with an insulin pump. The sera of diabetic patients contained on average a fivefold higher insulin concentration than did control sera. However, addition of 300 mU/litre insulin to sera that contained on average 30 mU/litre did not improve the development of cultured embryos. The absence of malformations after culture in sera of diabetic women may relate to the fact that their blood glucose control was on average reasonably good.