The adipokinetic hormone (AKH) of the locust has been isolated, characterised and synthesised1,2. In this insect, its function is to mobilise diglycerides from the fat body to provide energy for sustained flight3. In addition, it modifies the metabolic activity of the flight muscles so as to oxidise fatty acids preferentially4. We report here another effect of AKH on locust metabolism, the direct inhibition of protein synthesis by the fat-body. This inhibitory activity may function in the locusts' overall adaptation to flight and it may also account for the presence of AKH both in larval locusts5 and in other species which do not fly for prolonged periods6.