The carbamazepine (CBZ) concentration of breast milk was determined in 19 epileptic patients of whom 13 were receiving CBZ monotherapy. Two lactation periods under CBZ monotherapy were followed in 2 of these 13 patients. The CBZ concentration of the breast milk was 36.4% of the maternal CBZ serum level (mean value). The correlation coefficient was 0.44; the regression line follows the equation y = 0.12X + 1.63. The CBZ concentration was measured in a total of 50 milk samples during the lactation period (cases of monotherapy plus combination therapy). The CBZ concentration ranged from 1 to 4.8 micrograms/ml (mean 2.5 micrograms/ml). The free CBZ concentration in the maternal serum was measured in four patients. The correlation between CBZ in the breast milk and free CBZ levels was 1.6 to 1.8. The ratio of carbamazepine-epoxide (ECBZ) in the milk to ECBZ determined in serum was 0.53. Continuing galactorrhea after delivery was observed in one patient. The CBZ and ECBZ levels determined 3.5 years after delivery were higher in the milk than in the serum. The observations of breast-fed and non-breast-fed children whose mothers were on CBZ monotherapy disclosed poor suckling in only 1 of 15 infants.