The effect of circadian rhythm on cyclosporine pharmacokinetics was studied in rabbits after oral administration of 10 mg/kg dose of cyclosporine at 10:00 a.m. and 10:00 p.m. The blood concentration data were subjected to simultaneous computer nonlinear least squares regression analysis using a 1-compartment pharmacokinetic model. The blood concentrations of cyclosporine at 10:00 a. m. were increased significantly during 2-6 hr compared to those at 10:00 p.m. The area under the blood concentration-time curve (AUC) and peak concentration $(C_{max})$ of cyclosporine at 10:00 a.m. were increased significantly compared to those at 10:00 p.m. The mean total body clearance (CL) of cyclosporine at 10:00 a.m. were decreased significantly compared to those at 10:00 p.m. It is reasonable to consider individual circadian rhythm for effective dosage regimen of cyclosporine in therapeutics.