Rate of oxygen consumption was used as a method of measuring the metabolic rate of flying locusts. The animals flew into an airstream from a wind tunnel while tethered to a balance so that lift could be determined. Wingbeat frequency was also recorded. The rate of oxygen consumption during flight was found to be determined principally by wingbeat frequency, but using mean values for steady flight good straight line correlations were also found between rate of oxygen consumption and lift, and wingbeat frequency and lift. However, the relationships between these three variables were seen to change during the early part of flight, when lift could be maintained while, in general, wingbeat frequency and rate of oxygen consumption declined. As wingbeat frequency decreased the amount of oxygen used with each wingbeat was found to remain fairly constant, but the amount of lift generated by each wingstroke increased. Therefore the proportion of the locust's total power output which was used to perform aerodynamic work was greater at the lower wingbeat frequency. It was considered that the action of adipokinetic hormone and the change from predominantly carbohydrate to lipid metabolism might be involved in the onset of the period of more economical, steady flight. However, no firm evidence of this was obtained from the experiments which were carried out. When locusts were injected with corpus cardiacum extract 1h before flight, their wingbeat frequency dropped more rapidly than that of saline injected animals. Injection of adipokinetic hormone immediately before flight had a detrimental effect on the locusts' flight performance.