The action of fenoterol, a uterine relaxant of the adrenergic beta-mimetic group of drugs, has been investigated on placental, myometrial, and cardiac blood pools. 113mIndium was injected intravenously and the blood pools were measured by recording gamma activity externally. In eight cases with oxytocin-induced labor, fenoterol inhibited the typical blood pool changes of labor, i.e., contraction-synchronized decrease of placental and myometrial blood pools with increase in cardiac blood pool. Furthermore, fenoterol produced an increase in placental and myometrial pools with corresponding decrease of cardiac blood pool. These changes were observed consistently. In four other cases, of which three had spontaneous labor, similar effects were observed. The possible mechanisms of hemodynamic action of fenoterol, with reference to uterine relaxation, are discussed.