Both physicians and the lay public raise questions about drug excretion in breast milk. Enhanced interest is seen with the increase in the number of mothers who wish to breast feed. 'Contamination' of breast milk by drugs and environmental chemicals imposes risks, both known and theoretical, to the infant. Drug information centres and physicians find a meagre amount of knowledge on which to base recommenda­ tions and the data which are available are often anecdotal or derived from single case reports. A critical look at both the nature and scope of our knowledge on this matter was undertaken for this review. This purpose was best served by the cooperation of pharmacologists and clinicians who have a special interest in certain classes of drugs. A unified pharmacokinetic approach was formulated to facilitate the evaluation of previous data and to offer a model for future studies. This work was, in part, published in Clinical Pharmacokinetics Vol. 5 No. I 1980. Its reception prompted this revised and expanded version and it is hoped that its critical analyses and comprehensive literature citations will foster more concerted and thorough research into drug excretion in breast milk; benefits would then accrue to both maternal and child health. This work is dedicated to Dr William A. Silverman with whom I was most for­ tunate to share experiences as a fellow in neonatology and whose dedication to paediatrics and clinical investigation in subsequent years, I have watched from afar.