i pAtlas of Histology of the Juvenile Rati should be of interest to toxicologic pathologists, toxicologists, and other biological scientists who are interested in the histomorphology of juvenile rats. For several decades the laboratory rat has been used extensively in nonclinical toxicology studies designed to detect potential human toxicity of drugs, agrochemicals, industrial chemicals, and environmental hazards. These studies traditionally have involved young adult rats that are 8-10 weeks of age as studies are started. It is becoming increasingly apparent that children and young animals may have different responses to drugchemical exposures, therefore, regulatory agencies are emphasizing toxicology studies in juvenile animals. p pWhile the histologic features of organs from young adult and aged laboratory rats are well known, less is known about the histologic features of organs from juvenile rats. Final histologic maturity of many organs is achieved postnatally, thus immature histo