Insulin binding to trophoblast plasma membranes and the placental glycogen content were measured in twelve healthy women, in eleven well-controlled gestational diabetic women who were treated either with diet alone (n = 4) or with insulin (n = 7) and in 18 women with well-controlled overt diabetes mellitus (six White B; four White C; eight White D). The competitive binding assay was carried out with 22 concentrations of unlabelled insulin. Binding data were analysed by a non-linear direct model fitting procedure assuming one non-cooperative binding site. Maximum specific binding was unchanged in the total collective of gestational diabetic women, but was decreased by 30% in those treated with diet (6.2 +/- 2.2%) and increased by 90% in insulin-treated women (16.4 +/- 10.2%) as compared to the control subjects (8.7 +/- 2.5%). The diet-treated women had only 40% as many and those treated with insulin had more than twice as many receptors compared to control subjects on a per mg protein basis and if expressed per total placenta. In patients with overt diabetes mellitus maximum specific binding (18.5 +/- 10.6%) was higher (p less than 0.05) due to more receptors compared to control subjects but was similar to the insulin-treated gestational diabetic patients. Maximum specific binding and receptor concentrations did not correlate linearly with maternal plasma insulin levels. Receptor affinities were virtually similar in all groups (1.8 x 10(9) l/mol). The placental glycogen content was reduced (p less than 0.05) to about 80% of that of control subjects in the diet-treated collective, whereas it was unchanged compared to control subjects in the insulin-treated gestational diabetic women despite a 40% increase (p less than 0.001) of the maternal-to-cord serum glucose ratio. In overt diabetic patients the maternal-to-cord serum glucose ratio and the placental glycogen content were higher (p less than 0.05) than in the control subjects. We conclude that trophoblast plasma membranes from gestational diabetic women treated with diet alone express less and those from women treated with insulin express more insulin receptors than those from a healthy control group in vitro. These differences could not have been disclosed without consideration of the mode of treatment. Trophoblast plasma membranes from overt diabetic women have more insulin receptors than those from healthy control subjects.