Glycogen reserves in the fat body of Locusta migratoria decrease dramatically during the first two hours of flight. In fat body of rested locusts only 10% of glycogen phosphorylase occurs in the active form. The enzyme is activated significantly during flight, when up to one-third of the total phosphorylase becomes active. Phosphorylase activation can also be accomplished by injection of corpus cardiacum extracts, to give a maximum of circa 75% active enzyme. Locust fat body is shown to contain protein kinase activity, which can be activated both by cyclic AMP and cyclic GMP. Apparent K a- values are 0.13μM for cyclic AMP and 0.16 μM for cyclic GMP. Results are discussed in relation to regulation of substrate utilization during flight.