The action of buphenin, a uterine relaxant from the group of adrenergic beta-mimetics, was examined on the blood pools of placenta, myometrium and heart, in 10 pregnant women. The method consisted of an i.v. injection of the radioisotope 113m-Indium and of registering the blood pools by external measurement of the gamma-activity. buphenin caused an increase in placental and myometrial blood pools with a corresponding decrease in cardiac blood pool. These changes were observed in all the cases examined and occurred whether the uterus had previously been inactive or in labour. The changes in blood pool were dependent on the dose of buphenin administered but there was no relationship apparent between the degree of pool changes and severity of pregnancy complication prevailing.