There is a major controversy over the relative value of anti-hypertensive drugs in hypertension in pregnancy. Our purpose was to study two different beta-adrenolytic drugs, atenolol, a cardioselective beta blocker, and labetalol, an alpha-beta blocker. Fifty-six hypertensive (BP greater than 140/90 mmHg) pregnant women were treated either with atenolol or labetalol. The patients were divided into two subgroups for which there were no statistical differences with regard to age, number of previous pregnancies, initial level of blood pressure and uricemia, proteinuric pre-eclampsia, beginning of therapeutic trial and delivery. The average daily dosage was 144.6 +/- 47.8 mg day-1 with atenolol and 614 +/- 47.8 mg day-1 with labetalol. This study shows: the same anti-hypertensive effect of the two drugs with control of blood pressure in 82% of the cases; a birth-weight significantly higher with labetalol (3280 +/- 555 g) than with atenolol (2750 +/- 630 g) (P less than 0.001); two still-births with atenolol; no adverse effects of the drugs during pregnancy and the neo-natal period; the trans-placental passage of atenolol and labetalol as shown by plasma dosages in the mothers and the new-born. It is concluded that atenolol and labetalol are safe and they are usually effective in the control of the hypertension complicating pregnancy. But labetalol appears to be better able to prevent the appearance of fetal growth retardation.