The transfer of labetalol into human breast milk and amniotic fluid was studied in women with pregnancy hypertension. The women were treated with labetalol 600-1200 mg daily. The ratio between the areas under the milk and plasma concentration versus time curves varied between 0.8 and 2.6. No consistent relation between milk and plasma concentration in the mother was observed either within the individuals during a dose interval or between different individuals. One of the nursed infants at the end of the dose interval had a plasma labetalol in the same range as the mother, and in another infant the level was below the detection limit. Amniotic fluid concentrations 2-3 h after dosing were generally lower than in plasma.