Bundles of 20-30 fast muscle fibres were isolated from the abdominal myotomes of the short-horned sculpin (Myoxocephalus scorpius L.). The energy cost of contraction was measured during oscillatory work at 4 ~ and 15 ~ following treatment with iodoacetate and nitrogen gas to block glycolysis and aerobic metabolism. Isolated fibres were subjected to sinusoidal length changes about in situ resting length and stimulated at a selected phase in the strain cycle. Preliminary experiments with untreated preparations established the strain amplitude and stimulation parameters required to maximize work output over a range of cycle frequencies at 4 ~ and 15 ~ Following oscillatory work, treated preparations were rapidly frozen, freeze-dried and the concentrations of phosphocreatine (PCr), creatine, adenosine 5'-triphosphate (ATP), adenosine 5'-di-and mono-phosphate and inosine 5-monophosphate measured by high performance liquid chromatography. The concentration of PCr declined in proportion to the total work done for up to 64 cycles without a significant change in ATP. Maximum power output was produced at acycle frequency of 5 Hz at 4~ (14-18 W/kg) and 17 Hz at 15 ~ (23 -27 W/kg). The rate:of utilization of PCr per cycle was independent of temperature. However, since work per cycle was higher at 4~
(2.7-3.TmJ/g wet weight) than 15~ (1.2-1.6 mJ/g wet weight), theenergetic cost of contraction decreased with increasing temperature.