To investigate age differences in mechanical power output of human muscle the force-velocity relationships have been examined on 87 males consisting of 4 different age groups: 23, 20, 21, and 23 subjects for the ages of 10, 13, 15, and 19 years respectively. With the use of a modified Wilkie's apparatus the velocities were measured at right angle during elbow flexion by after-load method. The loads applied were 10 to 60% of the maximum isometric strength. The maximum shortening velocity was also measured with no load. All the data were fitted to Hill's characteristic equation. (P+a)(y+b) = (P_0 +a)b where P is load or force, V is velocity, P_0 is isometric strength, a and b are constants. The results obtained are as follows: 1) Force-velocity relations calculated after correcting the effect of inertia of the forearm and apparatus were well fitted to Hill's equation in different age groups (r >0.994). 2) The ratios of a/P_0, which characterize P-V relation, were almost constant (0.40-0.45) regardless of age, and thus the maximum power (PVmax) appeared under the load of about 35%P_0. 3) In the values of P_0, PVmax and the velocities under various loads their increments in 10-15 years of age were approximately 90% of total increment in 10-19 years of age. This increment (i.e., 90%) was found to be considerably less than that of simple reaction time quoted. 4) The velocities/forearm length have also developed with age, suggesting that the improvement in velocity at any load could not merely be due to the growth of muscle length, but due to the qualitative development in muscle function. 5) From these results, it was concluded that the age difference in muscle power should be attributed more to the muscular function rather than the nervous function.