The purpose of this study was to determining the range of aerobic exercise on a treadmill for male Wistar rats at different ages. Twelve male Wistar rats were divide in three groups of immature, adults, and old (n= 4 in each). At first session, the rats began to run at a rate of 2 m/min to perform the fatigue test, and the treadmill speed was increased by 2 m/min every 2 minutes. This process of acceleration continued until the rats were no longer able to continue moving on the treadmill and became exhausted. Then, blood lactate of each subject measured immediately and their maximum speed was recorded. After 48 hours of recovery, animal performed maximum recorded speed on a treadmill in three 10-minute steps of 25%, 50% and 75%, respectively. Immediately after each percent blood lactate were measured and recorded. Immature rats at an average speed of 18 m/min were reached to their maximum speed with an average lactate concentration of 8±1.8 mmol/l. Adults rats at an average speed of 36 m/min were reached to their maximum speed with an average lactate concentration of 6.8±0.4 mmol/l. The old rats reached their maximum velocity with an average of 30 m/min with an average lactate concentration of 6.95±0.9 mmol/l. Therefore, it recommended that aerobic exercise in untrained rats start at a light speed, i.e. 25% of their maximum speed, which is lower than the lactate threshold, and gradually continue up to 50% of their maximum speed