SummaryTo investigate the origin of insulin in amniotic fluid amniocenteses were carried out in pregnancies with live, dead and anencephalic fetuses. Amniotic fluid insulin of pregnant women bearing live fetuses was 9.0 ± 2.1 μU/ml; in six women with dead foetuses amniotic fluid insulin was not detected. A significant positive correlation was observed between gestational age and the amniotic fluid concentration of insulin. In the amniotic fluid of the four women bearing anencephalic fetuses, the amount of hormone was within normal limits (10.0 ± 1.4 μU/ml). Intravenous glucose administration (0.33 g/kg body weight) to the mother does not influence levels of insulin in amniotic fluid, but brought about changes in amniotic fluid glucose concentration. These findings support the conclusion that human amniotic fluid insulin is of fetal rather than maternal ori