INFOBOX By determining the pharmacokinetics of marker substances whose absorption characteristics correlate with GI functions, an indirect observation of physiological GI conditions is possible. Paracetamol and caffeine can be used for monitoring the kinetics of the gastric emptying of liquids. Additionally, caffeine is a feasible marker for the monitoring of in vivo disintegration times of orally applied dosage forms. For the monitoring of the emptying of solids from the stomach, the sulfamethizole capsule method has been proposed. The sulfasalazine/sulfapyridine method can be applied to determine orocecal transit time. All of these methods have in common the fact that they do not influence the GI physiology itself, no specialized equipment is needed, and they can be applied under more physiological conditions.