To gain a fuller appreciation of the toxicological significance of chemically induced histopathological changes in an organ, it is helpful to have an understanding of changes in the biochemistry and physiological function at the target site following chemical exposure. These biochemical changes are often reflected in deviation from normal levels of constituents derived from the target organ present either in the circulating blood or excreted in the urine. The measurement of these biochemical constituents gives useful evidential clues of the onset, progress and repair of chemical injury in the intact animal. This information is of value in the design of toxicological studies for the risk assessment of a chemical in experimental animals and in the extrapolation of animal data to man. Furthermore, the experimental design requires a fuller understanding of the route of exposure, the absorption, distribution, metabolism and excretion of the test compound and the cascade of biochemical changes set in train in the target organ following chemical insult.