The specific aims of this study are to determine the effect of increasing amounts of grapefruit juice on oral cyclosporine pharmacokinetics (clearance) and pharmacodynamics (toxicities). Fifty stable renal transplant patients who are maintained on a constant oral cyclosporine maintenance dose will be enrolled in the study. Differences in pharmacokinetic parameters (volume of distribution, half-life, and clearance) between cyclosporine alone and cyclosporine with grapefruit juice will be assessed.