Drugs administered to the maturing host may (a) be used to alleviate diseases peculiar to his age group, (b) undergo alterations in metabolic fate and distribution to influence therapeutic effectiveness as compared with adults, or (c) show unexpected adverse effects as a result of systems placed at risk during development. Pharmacologic treatment of neonatal "physio logic " hyperbilirubinemia has been studied, and drugs undoubtedly will be designed for a specific effect on other diseases characterized by a slow rate of enzyme (or transport mechanism) maturation. Many developmental phar macology studies have considered the effect of host physiology on actions of a drug or, conversely, the effect of a drug on the host. For example, failure in drug efficacy may be secondary to a low conversion of the parent drug to active metabolite (host effect on drug) or structural malformations may be the result of drug action on a developing tissue (drug effect on host). Dif ferences in clinical response or potential toxicity for a drug are demon strated by comparative studies in the adult versus the young. Of more im portance, however, is an explanation for the observed age-related differ ences in drug action. Such explanations are often broadly applicable to in vestigations with adults and enhance rational therapeutics in children. Un fortunately, more age-related drug actions have been described than ex plained. Developmental pharmacologists must now accept the challenge to build programs that encourage observation and explanation of drug-host in teractions on an age continuum (1).This review will cite studies to illustrate the three areas of developmen tal pharmacology outlined above. The effect of enzyme inducing agents on hyperbilirubinemia will be considered as an example of drug use for an ill ness commonly found in children. The development of liver mixed function oxidase activity as influenced by hormones will illustrate modification of drug fate by age of the host. Brief reference to tetracycline effects on teeth 1 This monograph was prepared with support from the Clinical Pharmacology