The concentration of "real" and "apparent" (= "real" hydralazine + acid-labile hydrazones) hydralazine in maternal and umbilical plasma obtained at delivery of 6 women treated with hydralazine and atenolol for pregnancy hypertension were measured by gas chromatography. In one of the patients, the concentrations of the same substances were subsequently measured in breast milk. "Apparent" hydralazine reached higher levels in umbilical than in maternal blood. The concentration of "real" hydralazine seemed to be at least as high in the fetus as in the mother. On the other hand, even though the fraction of "real" (i.e. presumably active) hydralazine was greater in milk than in plasma, the total concentration was smaller, and the estimated dose per milk feed of 75 ml would not exceed 0.013 mg. Thus, hydralazine treatment of the pregnant woman would expose her fetus to effective concentrations of the drug, but breast feeding would not result in a clinically relevant concentration in the infant.