The purpose of this study was to determine pharmacokinetic parameters and tissue distribution patters of urinary trypsin inhibitor(UTI) in Sprague-Dawley rats. $Na^{125}$I was conjugated to UTI to make $^{125}I-UTI$ and the concentrations were determined by $gamma$-counter. With the aid of nonlinear least-square regression analysis for i.v bolus injection of 1,000 unit UTI including $^{125}I-UTI$, the temporal concentration curves were best fitted by 2-compartment open model. The distribution phase half-life was 0.39$pm$0.02 hours whereas the elimination half-life was 12.99$pm$1.05 hours in male rats. The volume of distribution and total body clearance in male rats were 0.28$pm$0.01 1/kg and 83.16$pm$1.15 ml/kg/h, respectively. We could not find any difference of pharmacokinetic parameters of UTI between male and female rats. UTI were distributed widely in rat organs. In both male and female rats, the kidney was the highest distributed organ. Amount of UTI in 24 hour cumulative urine in male rats was 36.22$pm$8.74% and that in 48 hours was 43.32$pm$10.55%. Excretion via feces was very scanty, with the 24 hours cumulative amount being only 2.76$pm$0.97%. This data suggest the main excretion route of UTI is urine.