Processes controlling the absorption, distribution, metabolism, excretion, and pharmacologic effects of drugs are likely to be immature or altered in neonates and infants. Absorption may be affected by differences in gastric pH and stomach emptying rate. Low serum protein concentrations and higher body water composition can change drug distribution. Drug metabolism enzyme activity is typically reduced in the neonate, but rapidly develops over the first year of life. Renal excretion mechanisms are low at birth, but mature over a few months. Limited data are available on the pharmacodynamics of drugs; infants show greater sensitivity to d-tubocurarine. Developmental changes are rapid during the first weeks and months of life, thus requiring continual modification of drug dosage regimens designed for treating pediatric patients.