The disposition kinetics of Cyclosporine A (CyA) in rat, based on measurement in arterial blood, appeared dose-linear over a wide i.v. dose range (1.2-30 mg/kg). Physiologically based pharmacokinetic (PBPK) analysis, however, demonstrated that this was an apparent observation resulting from counterbalancing nonlinear factors, such as saturable blood and tissue distribution, as well as clearance (CLb). A PBPK model was successfully developed taking into account these multiple nonlinear factors. Tissue distribution was distinctly different among various organs, being best described by either a linear model (muscle, fat; Model 1), one involving instantaneous saturation (lung, heart, bone, skin, thymus; Model 2), noninstantaneous saturation (kidney, spleen, liver, gut; Model 3), or one with saturable efflux (brain; Model 4). Overall, the whole body volume of distribution at steady state for unbound CyA (Vuss) decreased with increasing dose, due at least in part to saturation of tissue-cellular cyclophilin binding. Clearance, essentially hepatic, and described by the well-stirred model, was also adequately characterized by Michaelis-Menten kinetics, Km 0.60 microgram/ml. In model-based simulations, both volume of distribution at steady state (Vss,b) and CLb varied in a similar manner with dose, such that terminal t1/2 remained apparently unchanged; these dose responses were attenuated by saturable blood binding. CyA concentration measured in arterial blood was not always directly proportional to the true exposure, i.e., unbound or target tissue concentrations. The PBPK model not only described comprehensively such complicated PK relationships but also permitted assessment of the sensitivity of individual parameters to variation in local nonlinear kinetics. Using this approach, dose-dependent CyA uptake into brain was shown to be sensitive to both active and passive transport processes, and not merely the affinity of the active (efflux) transporter at the level of the blood-brain barrier.