Measurements of concentrations for drugs from plasma or biological fluids taken together with the clinical status of the newborn infant may provide better assurance of avoiding potentially dangerous over- or underdosing. They are also useful as guidelines in requisite clinical trials and study designs. To the developmental biologist and pharmacologist, they serve as potent biological probes for various events underlying biochemical differentiation or maturation in the human fetus and newborn infant. Future research developments in neonatal drug monitoring should include improvement of micro-methods for drug assays and refinement of noninvasive techniques for monitoring drug concentrations from biological fluids and for evaluation of drug effects.