The purpose of this study is to observe the changes in ^99mTc-MDP uptake of the tibia during blood-flow and skeletal phases and determine the clear borderline between both phases. Serial ^99mTc-MDP uptake measurements were performed, as control, in ten matured rabbits with skin incision only and in thirty matured rabbits with fractures of middle and distal one-third of tibia, at ten minutes, thirty minutes, one hour, two hours and four hours after injection of bolus. A value in uptake ratio was calculated by measurements of ^99mTc-MDP uptake at each observation time for analogous regions of normal and fractured tibia applied with or without tourniquet. The results obtained were as follows: 1. Group I (skin incision only in ten rabbits) 1) Without tourniquet, the mean ^99mTc-MDP uptake ratio tibia was value of 1.05 at ten minutes, 0.97 at thirty minutes, 0.99 at one hour, 1.01 at two hours, 0.97 at three hours and 1.08 at four hours. 2) With tourniquet, the mean ^99mTc-MDP uptake ratio of tibia was value of 0.61 at ten minutes, 0.64 at thirty minutes, 0.76 at one hour, 0.98 at two hours, 0.94 at three hours and 1.05 at four hours. 2. Group II (fracture with K-wire fixation in twenty rabbits) 1) Without tourniquet, the mean ^99mTc-MDP uptake of tibia was value of 0.73 at ten minutes and 0.80 at two hours. 2) With tourniquet, the mean ^99mTc-MDP uptake ratio of tibia was value of 0.45 at ten minutes and 0.76 at two hours. 3) There were no significant change of the ^99mTc-MDP uptake ratio in two to four hours after fracture whether the tourniquet was applied on or not. 3. In cases with tourniquet application, there were no significant changes of ^99mTc-MDP uptake ratio during two to four hours in group I anda II. This experiments suggest the optimal time for evaluation of the skeletal phase is in two hours following the injection of bolus (^99mTc-MDP).