The action of the uterine relaxant ritodrine on placental, myometrial and cardiac blood pools has been examined during spontaneous and oxytocin-induced uterine activity. 113mIndium was injected intravenously and the blood pools were measured by recording gamma-activity externally. After i.v. injection of Ritodrine, the typical blood pool changes in labor were inhibited. Thereafter, placental and myometrial blood pools increased more than due to inhibition of labor alone. Simultaneously, there was a corresponding decrease in cardiac blood pool indicating an increase in cardiac output. The rapid inflow of blood in the uteroplacental region after Ritodrine indicates an improved hemodynamic situation. This action could be of benefit in emergency situations in a hyperactive ischaemic uterus during labor.