A new approach has been developed to determine the factors that influence the balance of drug elimination from the body. This approach is based on 1. a six-compartment-model with compartments connected by different flow rates assuming kinetic processes of first order, 2. on solutions of geometric series and 3. on numerical solutions of a system of non-linear equations. In the model, different ways of drug elimination have been considered: renal, liver and fecal elimination of the drug and its metabolism in the liver. The organs have been characterized by their drug availabilities. Further, the metabolic activity of the liver, the efficiency of drug absorption and re-absorption from the gastrointestinal (GI) tract have been included. This paper identifies three events, characterized by their efficiencies--1. hepatic excretion, 2. elimination of drug from liver into the gall bladder in its non-metabolized form and 3. the re-absorption of the drug from GI tract--as necessary conditions of enterohepatic circulation of (EHC). The product of these efficiencies has been introduced as the efficiency of enterohepatic circulation. Further, the drug bioavailability as a function of the efficiency of EHC is presented. The study shows that--based on the total amounts of non-metabolized drug in urine after p.o. and i.v. administration to animals with and without cannulated bile duct and in the bile of cannulated animals--the efficiency of EHC, bioavailability of the drug, renal and hepatic availability of the drug, metabolic activity of the liver and efficiency of drug absorption and re-absorption from the gut can be determined. Additionally, it has been shown that, depending on the efficiency of enterohepatic circulation, small variabilities in drug pharmacokinetic properties can cause high variance of drug bioavailability. The publication points towards the efficiency of EHC as on a factor that plays a key role in establishing in vitro-in vivo correlation.