Existing experimental strategies for the in vivo evaluation of factors affecting oral bioavailability have been reviewed. Based on concepts that have evolved, an integrated set of strategies emerges that appears capable of providing estimates of the individual contributions attributable to absorption, losses in the gut lumen, and first-pass elimination in the gut wall and the liver. The only assumptions are linear pharmacokinetics and constant clearance between treatments. These methods are also suitable for the assessment of metabolite bioavailability after drug administration and the quantitative determination of sites of biotransformation and metabolite formation in vivo.