The pharmacokinetics of cyclosporine, a potent immunosuppressive agent, are described for rats given 5 mg/kg iv, measuring blood concentrations by a specific HPLC method. Cyclosporine followed first-order kinetics and blood concentrations vs. time data were adequately described by a three-compartment open model system. The mean half-life of the lambda 1 phase was 0.11 hr, that of the lambda 2 phase was 1.82 hr, and the half-life of the estimated lambda 3 phase was 23.79 hr. The mean apparent volume of distribution and Vss were 5.69 and 4.54 liters/kg, respectively. The mean blood total body clearance was 3.38 ml/min/kg. The literature was reviewed to obtain comparable data from other mammalian species, and satisfactory kinetic information was found for humans, dogs, and rabbits. Interspecies variants in physiological parameters and cyclosporine pharmacokinetics were considered and treated as a property and consequence of body size (allometry), nullifying anatomical and physiological differences between species. A relationship between pharmacokinetic time (a variable in terms of chronological time) and body weight was found. It is suggested that the metabolic capacity (based on liver weight) to eliminate cyclosporine is similar in humans, rabbits, and rats, whereas the dog showed a potentially double capacity to metabolize the drug. However, metabolic and pharmacodynamic studies are also needed to predict toxicity accurately among species.