A physiologic pharmacokinetic model of cyclosporin has been developed in the rat aimed at predicting the time course of drug concentrations in blood, organs, and tissues. The model assumes that tissue distribution is perfusion-rate limited and that each tissue acts as a well-stirred compartment. The unbound equilibrium distribution ratios as well as the values of the fraction unbound and the distribution isotherm of cyclosporin between erythrocytes and plasma are included in the rate equations describing the time course of the drug concentration in each tissue. Parameter values for the rat were obtained experimentally from a continuous infusion study, in which 2.7 and 13.9 mg/kg per day doses of cyclosporin were administered subcutaneously to each of two groups of rats by osmotic pumps for 6 days. Steady-state cyclosporin concentrations in blood, CSF, and 18 different organs and tissues, were determined by a monoclonal antibody RIA. Differences in values of the unbound equilibrium distribution ratios in some tissues and unbound clearance indicated that both the processes of distribution and elimination may have elements of nonlinearity over the range of dosing rates tested. The model was evaluated in the rat with a kinetic experiment in which a 6-mg/kg dose of cyclosporin was infused intravenously over 15 min, with measurements of blood concentrations until 56 hr. Good agreement was obtained for the volume of distribution at steady state (blood), Vss, between the perfusion model and that calculated from the kinetic experiment. Also, the model prediction of the blood concentration temporal profile agreed closely with that observed except in the early moments, when distribution out of blood occurred considerably slower than predicted. On scaling the model up to humans, good agreement was found between the predicted plasma concentration-time profile and Vss and experimental data from the literature. Both rat and human data suggest that partition into adipose tissue plays an important role in the pharmacokinetics of cyclosporin.