An alternating cycle of eccentric-concentric contractions in locomotion represents a sequence when storage and utilization of elastic energy takes place. It is possible that this storage capacity and its utilization depends on the imposed stretch loads in activated muscles, and that sex differences may be present in these phenomena. To investigate these assumed differences, subjects from both sexes and of good physical condition performed vertical jumps on the force-platform from the following experimental conditions: squatting jump (SJ) from a static starting position; counter-movement jump (CMJ) from a free standing position and with a preparatory counter-movement; drop jumps (DJ) from the various heights (20-100 cm) on to the platform followed immediately by a vertical jump. In all subjects the SJ, in which condition no appreciable storage of elastic energy takes place, produced the lowest height of rise of the whole body center of gravity (C.G.). The stretch load (drop height) influenced the performance so that height of rise of C. of G. increased when the drop height increased from 26-62 cm (males) and from 20-50 cm (females). In all jumping conditions the men jumped higher than the women. Examination of the utilization of elastic energy indicated that in CMJ the female subjects utilized most (.simeq. 90%) of the energy produced in the prestretching phase. Similarly, in DJ the overall change in positive energy over SJ condition was higher in women as compared to men. Although the leg extensor muscles of the mean subjects could sustain much higher stretch loads, the females may be able to utilize a greater portion of the stored elastic energy in jumping activities.