White muscle fibres from dogfish were used to compare the energetic costs of shortening by fully active muscle and by relaxing muscle. The muscle preparation was tetanized for 0.6s and shortened either during stimulation or during relaxation. The distance shortened was 1mm (about 15% L0, the muscle length optimum for force) and the velocity was 3.5 or 7.0mm s−1 (about 15 or 30% V0, the maximum velocity of shortening). Isometric tetani at L0 were also investigated. Mechanical work and heat production were measured, and work + heat was taken as a measure of energetic cost. Both work and the energetic cost were higher with shortening during stimulation than with shortening during relaxation. The results suggest that shortening during relaxation, which is known to occur during locomotion in vivo, may be an energy-saving strategy.