The plasma renin activity (PRA) was measured in 26 pregnant patients before and during ritodrine infusion (10 patients), during isoxsuprine infusion (12 patients), and during glucose infusion (4 control patients). All patients were in the third trimester of pregnancy, not in labour and not hypertensive. A low dose of ritodrine, a beta2 adrenergic agent, caused a rise of PRA but no simultaneous rise in heart rate; this suggests that beta2 activity is involved in renin release. Normal doses of ritodrine and isoxsuprine increased the maternal heart rate to 110 to 120 beats/minute and also doubled the PRA level. Two hours after the end of the ritodrine infusion the PRA level was still above that before the infusion. The results suggest high reserves of renin during pregnancy.