Most drugs have been found to be excreted in human breast milk. Usually when the drug is taken in therapeutic amounts for short periods of time by the mother, the levels of the drug in breast milk are sufficiently low to be of little hazard to the infant. However, is a breast-feeding infant should become ill or fail to thrive and the morbidity cannot be explained, one of the following should be done: 1. Discontinue the drug. 2. Discontinue breast feeding. Frequently this can be accomplished on a temporary basis with the mother pumping her breasts to maintain lactation while the response of the infant is monitored. 3. Collect maternal plasma, breast milk, and infant plasma samples for drug assay. In situations in which this can be accomplished, it may be possible to incriminate (or exonerate) a drug or one of its metabolites as the source of the morbidity on the basis of the amounts of drug found in the milk or the infant's plasma. As tedious and impractical as this approach may seem, it would eventually lead to the accumulation of a reasonable amount of data from which could be drawn sensible conclusions about the effect of drugs on the breast-fed infant.