Knowledge of pharmacokinetics (action of organisms on drugs) and pharmacodynamics (drug action on living organisms) allows for the proper assessment of the most suitable dose, dosing intervals, route of administration, as well as dose adjustment when clinically indicated. The basic physical scientific principles of the movement of drug particles across biologic barriers, their subsequent conversion to other chemical forms, and their elimination are reviewed in general terms. The specific metabolic pathways for aspirin and paracetamol (acetaminophen) are then discussed in more detail. Elimination of salicylate involves two saturable (nonlinear) major pathways and three apparently first-order (linear) minor pathways; these mixed order kinetics lead to somewhat complex mathematics affecting elimination half-life which, in turn, can have implications for anticipating side effects and toxicity. The kinetics of acetaminophen also involve various pathways, but studies have shown a good correlation between the expected and the observed elimination half-life of this drug. Comparison is made between the in vivo handling of the two analgesics, but it is stressed that these data apply only to healthy adults under normal conditions and cannot be extrapolated to patients with underlying disease processes.