The term "bioavailability" is used to describe the actual percentage of a drug released from the dosage form, which reaches the receptor site in sufficient quantity to induce a biological effect. In this connection there are many problems arising in conjunction with the formulation of the preparation, as well as through the interaction of physiological and pathological factors. Prerequisite to the bioavailability of an orally administered preparation is, firstly, the quick disintegration of the dosage form, and secondly, the release and dissolution of the active substance. No in vitro methods for the examination of these factors have as yet been evolved which would allow a reliable prediction of bioavailability in man. Animal experiments only permit limited prognoses, since numerous influential factors, such as absorption from the intestine, metabolism and metabolic rates, influence of physical and mental stress, etc., strongly dependent upon the species. Bioavailability is determined using pharmacokinetical techniques on the area affected by the dose-response curve, or a combination of these methods.