The effects on the fetus of the acute lowering of blood pressure in maternal hypertension with dihydralazine 6.25 mg were studied in 27 Black patients with diastolic blood pressures of 110 mmHg or more. The fetus was found to be growth-retarded in 13 of the 14 cases in which fetal heart rate decelerations coincided with a fall in maternal blood pressure. Only 2 of the 13 fetuses without heart rate changes were growth-retarded (P less than 0.001). Continuous fetal heart monitoring during the lowering of the blood pressure helped in recognizing the 'comprised' fetus if this had been missed by other observations.