A large volume liquid scintillation detector was used to determine rates of excretion, absorption, and intercompartmental clearance in ambulatory dogs. A γ-emitting test substance was administered intravenously, orally in aqueous solution, and orally in sustained and delayed-release dosage forms. Unanesthetized female dogs were partially restricted, catheterized, and fasted prior to dosing. Where applicable, plots of log per cent whole body retention as a function of time were resolved into linear components following apparent first-order kinetics. The rate constants of these components were calculated and compared. Sustained-release forms were prepared which exhibited zero-order release characteristics in vitro and in vivo , and the characteristics of an enteric coated dosage form were compared after in vitro and in vivo testing. The procedure provided such parameters as absorption, excretion, release, and the effect of formulation techniques and formula variation on the biological availability of the test substance employed without the necessity of excreta and blood sampling and analysis.