Current guidelines (CPMP Note for Guidance in Europe and FDA Guidance for Industry in the USA) consider a waiver of bioavailability/bioequivalence studies for immediate release dosage forms of highly soluble, highly permeable drug substances (Class I according to the BCS). In this paper, a waiver of BA/BE studies is being proposed also for Class III compounds (high solubility and low permeability) in fast dissolving products without excipients which may modify gastro-intestinal transit or membrane permeation. This type of drug substance may be an even better candidate for a waiver as, in this case, bioavailability will not so much depend on the formulation characteristics, as on drug substance properties (e.g. permeability).