In the last several years a large body of information has developed concerning renal function in the perinatal period. Such data provide the physiological and biochemical basis on which rational approaches to the use of drugs in premature and full-term newborns may be based. In this review the authors focus on two areas of research interest: renal hemodynamics and organic anion secretion. These subjects have received considerable attention because they are important in total renal function and because they are amenable to quentitative analysis in experimental animals. The final section of this review deals with the action of diuretics in the newborn, a subject of significant clinical importance, which has only recently begun to receive quantitative experimental analysis.