Tacrolimus is an immunosuppressant drug used for the prophylaxis of organ rejection in patients who receive allogenic liver or kidney transplants. This study investigated the relationship between tacrolimus blood concentration-time profiles after 3-, 7-, and 10-mg single oral doses were given to 18 healthy, drug-free, nonsmoking, institutionalized male volunteers. The protocol used a single-dose, 3-period, 3-treatment, nonmasked, randomized-block, complete crossover design. The 90% CIs of the ratios of dose-adjusted, mean, logtransformed values of maximum blood concentration, area under the tacrolimus blood concentration-time curve (0 to the last measurable concentration; lower limit of quantitation = 0.5 ng/mL), and area under the blood concentration-time curve (0 to ∞) fell within the range of 125%, indicating dose proportionality for these parameters under experimental conditions. Power to detect a 20% difference for the 3 doses tested was 82.2%, 66.7%, and 72.8%, respectively.