Eleven pregnancies were studied in which i.v. hydralazine was given within 24 hours of delivery. Hydralazine (with its active metabolites, the hydrazones) was measured in maternal, cord and neonatal blood and in breast milk by high performance liquid chromatography (HPLC). Cord venous hydralazine concentrations Were approximately equal to maternal venous concentrations. Hydralazine was detectable in breast milk and concentrations were about half those in maternal blood taken simultaneously. Concentrations of hydralazine Were 557 nmol/l and 293nmol/l in the serum of two babies after breast feeding. Boiling expressed breast milk destroyed hydralazine. One other infant, with coarctation of the aorta, required 6 mg/kg/day of hydralazine to lower his blood pressure and his plasma concentration exceeded 8000 nmol/l. We conclude that the transfer of hydralazine across the placenta and into "reast milk is unlikely to subject neonates to pharmacological effects.