In the recent past, drug-induced morphological changes in animals and man have been reported for a wide variety of agents. Descriptions of severe changes, more appropriately termed toxicities, have been quite valuable in clinical areas (for the prediction of dose-limiting toxicological problems) and in drug development areas (for evaluation of the relative efficacy of analogs). It has now become clear that such purely descriptive approaches to drug-induced morphological changes are not adequate. They may not help predict undesirable side effects, nor do they often suggest how to more safely utilize a drug. It has become necessary to study the mechanisms by which drugs induce morphological changes as a means of better understanding basic drug actions. Such information may make it possible to more rationally search for new agents that are more effective or have fewer undesirable side effects than those drugs currently available. Principle areas of effort are currently being directed toward understanding the mechanisms by which anthracyclines are cardiotoxic and platinum compounds are nephrotoxic.