In order to define the homeostatic mechanisms which affect the secretion of human chorionic somatomammotropin (hCS), its plasma levels were determined by a radioimmunoassay method in normal, prediabetic and chemically diabetic pregnant women in basal conditions and after stimuli. Oral glucose failed to modify hCS plasma levels, as did continuous i.v. glucose infusion. Rapid acute i.v. glucose decreased them, suggesting that only sudden changes in maternal plasma glucose can exert a regulatory effect on hCS secretion. I.v. arginine increased hCS levels, new evidence of the great similarity of structure and function between growth hormone and hCS.