The effects of external loading, in the form of small weights distributed evenly over the limbs and torso, on physical performance and power output have been studied during vertical jumping in 10 children and four young adults and the results compared with maximal cycling. The results show under control (unloaded) conditions the absolute peak power output (W) achieved by children and adults was 572 W (45%) and 765 W (25%) respectively higher in cycling than jumping. The addition of weights during jumping served only to increase this difference. External loading produced a linear decrease of W in both groups of subjects. The reduction in W was entirely due to a decrease of take-off velocity (VT). The relationship between VT and added weights (delta wt) could be described by the equations: VT (ms-1) = 1.91 - 0.042 delta wt (kg); r = -0.96 (children); VT (ms-1) = 2.49 - 0.021 delta wt (kg); r = -0.99 (adults). Thus, contrary to the recent work of Caiozzo and Kyle (1980) which involved stair-climbing, body size and speed of movement in children and young adults would appear to be optimally matched for the production of lifting work during vertical jumping. External loading reduces the generation of power output immediately prior to take-off of a maximal jump from a force platform.