The excretion of drugs in breast milk has been appreciated for some considerable time and a review of the subject appeared as early as 1908 (cited by Illingworthl). Over the past 5 years a number of extensive reviews have appeared. Most noteable have been the monograph edited by Wilson 2, reference text edited by Briggs et al 3 and the statement issued by the American Academy of Pediatrics. 4 With increasing experience, it is becoming apparent that the number of drugs which are an absolutec ontraindication to breast feeding is relatively small. The collection of data on drug excretion in breast milk has largely, of necessity, been opportunistic. As a consequence much of the information has been derived from small numbers of patients or from anecdotal clinical observations. The reader is cautioned to bear this in mind when reading this and other reviews and original reports in the literature. The absence of reported adverse effects relating to a particular drug in breast fed infants does not imply that a drug is safe, only that adverse effects have not been reported in tile literature. The following article will review briefly the principles of the excretion of drugs in breast milk and discuss a number of important areas in relation to the transfer of drugs to the breast fed infant.