Ewes of between 126-134 days of gestation (term 147 days) were anesthetized and underwent vascular cannula placement in the maternal and fetal jugular vein and carotid artery. A flow probe was placed around the uterine artery supplying the pregnant horn. A median of 7 days after surgery, the effect of nitrendipine (NIT) (2-8 micrograms/kg/min in stepwise increments of 30 min each followed by a 2-h recovery period) on maternal and fetal systemic arterial blood pressure (BP), heart rate (HR), the uterine blood flow (UBF), and blood gas tensions was determined (n = 5 experiments). Control experiments (n = 4) in which vehicle only was infused were also performed. The infusion of NIT was associated with a small fall in maternal systolic BP and a progressive fall in maternal diastolic BP (p less than 0.01, less than 0.001; two-way ANOVA); UBF did not change. Fetal systolic BP actually rose during the NIT infusion (p less than 0.02) but there was no difference in fetal diastolic BP. Both maternal and fetal HR rose in the NIT group. Fetal pH remained very stable throughout as did PaCO2. Fetal PaO2 fell somewhat during the recovery stages in both NIT and control groups. There is considerable interest in the possible use of calcium channel blockers in treating pregnancy-induced hypertension. These preliminary experiments provide no evidence in conflict with such a use.