SUMMARYThe effects of daily administration of propranolol on the distribution of cardiac output were studied in the late pregnant guinea‐pig.The animals were given either saline, 5 or 50 mg/kg per day ofl‐propranolol for 15 days during the last trimester of gestation.Blood flow responses were assessed by injecting 15 μm microspheres into the conscious animal one to two days prior to parturition.The fraction of cardiac output reaching the placentas was significantly reduced in both groups (P<0.5 andP<0.001, respectively), while flow to all other organs studied remained unaltered.It is suggested that propranolol exerts a selective effect on the placental vascular bed in the guinea