In vivo predictability of results obtained from studies of drug metabolism using isolated rat hepatocytes is questionable, mainly because of modeling difficulties due to the simultaneously occurring substrate transferring processes. In the present study, an attempt was made at simplifying the models used to describe the kinetics of biotransformation by enzymes enclosed in a cellular environment. Viability assessment of the cell preparation indicated that the cell membrane was intact and functional. Six corticosteroids were used in these studies. Simplifying assumptions concerning uptake and protein binding were confirmed by running independent experiments. Progress curves of unchanged steroid disappearance from the cell suspending medium at different initial concentrations were used to either confirm applicability or detect deviations from simple Michaëlis-Menten behavior and were fitted to the appropriate kinetic models by means of nonlinear least-squares regression analysis. As an example, corticosterone extraction ratio obtained in this study compared well with literature values from intact rats. A linear correlation was found between the logarithm of the apparent first order rate constant (Vm/Km) obtained at low substrate concentrations and the logarithm of the oil/water partition coefficients of 17 alpha-hydroxyprogesterone, corticosterone and hydrocortisone.