The authors review the literature dealing with the comparison of oral multiple-unit and single-unit sustained- release formulations. Some elements of gastro-intestinal physiology and the fate of dosage forms in vivo are presented. The gastro-intestinal transit time and the variability of both formulation principles are then considered in order to explain possible differences. The spreading of multiple-unit dosage forms throughout the gastro-intestinal tract is also examined. Finally, the interest of pharmacokinetic parameters for the comparison of oral sustained-release delivery system is discussed.