Eleven hypertensive pregnant patients were treated with nifedipine orally 10 mg thrice daily. At steady state, nifedipine concentration, measured by the high-performance liquid chromatographic method, was 4.3 +/- 1.0 ng/ml (mean +/- s.e.m.) in maternal serum in the third trimester of pregnancy. During delivery, the parent drug levels in maternal and umbilical serum and in amniotic fluid were 12.4 +/- 4.0, 8.6 +/- 2.3 and 2.5 +/- 1.2 ng/ml, respectively. Small amounts of nifedipine were found also in urine of the neonates. In breast milk, the nifedipine concentration was 4.1 +/- 0.8 ng/ml on the third day after delivery. The antihypertensive effect of nifedipine did not correlate with the serum drug concentration. The outcome of pregnancy was favourable in all cases.