The plasma drug concentration-effect relationship after an oral dose of 100 mg metoprolol has been studied in 8 women in the third trimester of a pregnancy complicated by hypertension. The study was repeated 3-5 months after parturition when all but 2 women were normotensive. Systolic and diastolic blood pressures (SBP and DBP) were measured in the sitting position followed by the change in heart rate on exercise. The average peak plasma concentration of metoprolol was almost 4-times higher in the non-pregnant state. Despite this difference, the reduction in exercise tachycardia and resting SBP was only slightly more pronounced after delivery than during pregnancy. In relation to the plasma drug concentration, metoprolol had four-times and twice the effect on heart rate and SBP during pregnancy as compared to the post partum period. The altered chronotropic response to metoprolol during pregnancy may be due to increased sensitivity or altered function of the beta-adrenergic system.