The effect of cimetidine on the pbarmacokinetic parameters of cyclosporine (intravenous administration) were determined in 6 healthy volunteers (22-48 years old, 48-62 kg) by cross-over design. Cyclosporine and cyclosporine metabolites in whole blood were analysed by fluororescence polarization immunoassay (TDx-FLX). The blood concentrations of cyclosporine After pretreatment with cimetidine (200 mg bid, for 3days) were increased significantly at 8-12 hrs compared to the control (p<0.05). The ratios of blood concentrations of cyclosporine metabolites (M1, M17) to parent drug were decreased significantly at 8-12 hrs (p<0.05). Total body clearance (CL) was also decreased significantly (p<0.05), and area under the curve $(AUC,\%)$ was increased but not significant.