Beta-adrenoceptor blockade (beta-blocker) has recently been introduced for use in treating hypertension of pregnancy. The aim of this investigation is to determine the effects of beta-blocker (alprenolol and labetalol) on parturition and on the fetal cardiovascular and metabolic system in normotensive pregnant rats. The continuous administration of alprenolol to pregnant Wistar rats revealed the following: a small placenta intrauterine growth retardation delayed parturition intrauterine death (21.8%) morbidity (30.3%) and, hypoglycemia of neonates. In the cases which received the administration of both labetalol and alprenolol, the growth of fetuses was retarded. However, there was twice as much growth retardation of the fetuses treated with alprenolol. No other abnormalities were observed. The administration of alprenolol in pregnant rat uteri (in vitro) resulted in stimulation of myometrial contractility (oxytocic reaction) and lowered the c-AMP levels in the bath medium. However, when labetalol was administered, uterine activity was reduced in a dose-dependent manner (tocolytic reaction) with a concurrent rise in c-AMP levels in the bath medium. The effect of alprenolol on rat uterine blood flow using the thermocouple method resulted in a decrease of 18-30 percent but with labetalol there was only a slight decrease (0-5%).