The excretion of rhein, a cathartic with active metabolite from sennosides, was investigated in breast milk samples of 15 post-partum women for at least 24 h after the intake of a therapeutic dose (15 mg sennosides/day) of a standardized, fiber containing senna laxative (Agiolax). Rhein was analyzed by a HPLC-method with a lower limit of detection at 1 ng/ml rhein, taking into account a possible binding as monoglucuronide and monosulfate. Maximal concentrations up to 27 ng/ml and in 89% values below 10 ng/ml were measured. Related to the daily milk volume, 73% of the women excreted less than 2 ng rhein/day. Based on median values, 0.017% of the sennoside intake (calculated as rhein) was excreted in breast milk. The amount of rhein transmitted to the infant is therefore 0.3% of the rhein intake of the mother. This is far below the oral rhein dose necessary for inducing a laxative effect. Accordingly, none of the breast-fed infants (n = 8) showed any difference in stool consistency in comparison with the non breast-fed infants.