Diabetes of the mother during pregnancy induces structural and functional adaptations in the fetal endocrine pancreas. We have previously shown in our experimental rat model, that the impact of this abnormal intra-uterine milieu leads, in the adult offspring, to a disturbance of the glucose homeostasis and to the development of gestational diabetes. The aim of the present work is to investigate wether these functional differences can be explained by structural differences at the level of the endocrine pancreas. Therefore the size and the structure of the endocrine pancreas, as well as the contribution of the insulin-, glucagon-, somatostatin- and PP-cells, were investigated morphometrically in the adult youngsters of mildly and of severely diabetic mothers, since both display a disturbed glucose tolerance but with divergent characteristics. Also the adaptation of their endocrine pancreas to pregnancy was measured and compared to that of a control pregnancy. In the offspring of mildly diabetic mothers, the size of the endocrine pancreas and the distribution of the islets of Langerhans are normal. Also the doubling of the endocrine mass during pregnancy is similar to controls. The high proportion of A-cells, especially in relation to a normal B-cell mass and the low amount of PP-cells, might play a role in the impairment of the insulin response in these animals and in the development of gestational diabetes. In the offspring of severely diabetic mothers a clear hypertrophy of the endocrine pancreas is noted, which is mainly due to the presence of numerous small islets and which does not increase further during pregnancy. In these animals, the size of the endocrine pancreas and of the B-cell mass have reached `pregnant' values without pregnancy, which coincides with an exaggerated insulin output and peripheral insulin resistance, as during normal pregnancy. No further increase in islet mass is seen during pregnancy, which is associated with gestational diabetes.