The transfer of atenolol and metoprolol to human breast milk was studied in 7 lactating women with hypertension. Concentrations of atenolol and metoprolol in maternal plasma and milk and in plasma of the nursed infants were determined. Blood samples were obtained from the women and from the infants during a dose interval. Milk was collected repeatedly from the left breast but only twice from the right breast. The ratio between the area under the milk concentration versus time curve (AUCm) and the area under the plasma concentration versus time curve (AUCp) in the mother varied between 2.0and 3.1 for metoprolol and 1 . 1 and 3.1 for atenolol. The milk concentrations of metoprolol in the right breast were similar to those in the left breast, although the latter had been emptied more frequently. For atenolol the milk concentrations in the right breast were lower than in the left breast. The plasma concentrations of metoprolol and atenolol in the infants were negligible or below the limit of detection. Our results show that exposure of the infant to metoprolol can be minimized if nursing is not undertaken earlier than 3 -4 hours after dose intake.

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.