Use of the standard measure of renal function, glomerular filtration rate when corrected for body size, indicates fully functional renal capacity by one year of age, which remains relatively constant until the fourth decade of life, when it begins to gradually decline with advancing age. The active tubular secretion and reabsorption of cations and anions provide an occasion for drug interactions which are difficult to predict without knowledge of the exact mechanisms involved. Data support developmental changes in net tubular secretion for some substances. For digoxin, the larger ratio of digoxin clearance to creatinine clearance that is observed in children decreases during adolescence to the lower ratio observed in adults, and this decrement is better correlated with sexual maturation than with chronologic age. Thus for drugs with significant renal excretion of active drug or metabolite, the clarification of net renal tubular mechanisms would provide important clinical information.