The physicochemical and pharmacokinetic factors involved in transfer of drugs into breast milk are reviewed. Passage of drugs into milk can best be viewed as a two-compartment system. Various ratios of the drug concentration values in the two compartments and ratios of maternal-to-infant intake have been described. Knowledge of the limitations of these methods is necessary to properly interpret and apply the literature on drug excretion into breast milk. Factors involved in choosing a drug for a nursing mother are listed, a stepwise approach to minimizing transfer of drug to the infant is presented, and the literature on the excretion of specific drugs into milk is reviewed. Generally, drugs given to nursing mothers reach infants in much smaller amounts than drugs given to pregnant women. Decisions about nursing during drug therapy and the choice of drug therapy in a nursing mother should be based on the dosage and duration of therapy, age of the infant, quantity of milk consumed, experience with the drug in infants, degree of oral absorption of the drug by the infant, potential long-term effects, possible interference with lactation, and non-dose-related toxicities (e.g., potential allergic reactions). Too often, the mother's need for a medication is perceived as a reason to discontinue nursing. By understanding the principles of drug passage into breast milk and systematically evaluating the mother's needs, infant factors, and the data on specific drugs, clinicians can usually devise treatment plans that allow nursing while minimizing the risks to the infant.