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 2.0 and 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.