The glandular lobe of the corpus cardiacum of Locusta migratoria is characterized by the presence of one type of secretory cells containing secretory granules with a diameter of 2000–3000 Å, and of axons with small granules (diameter 950 Å). The axons form synaptic contacts with the cells. The secretory products of the glandular cells are released by exocytosis in the gland proper. In the axon terminals, release is indicated by the presence of synaptic vesicles and omega-shaped indentations. In order to study possible changes in the secretory activity of the gland during flight, three groups of locusts were studied with quantitative electron microscopical methods, i.e., resting insects, insects having flown for 5 min (short flight, SF), and insects after prolonged flight (PF, 60 min). After flight, greater secretory activity of the glandular cells is indicated by a marked increase in the number of exocytotic pits in SF and PF insects, by an enlarged nuclear volume as well as an increased amount of secretory products in the Golgi cisterns of PF animals. Flight also causes greater release activity in the axon terminals. From these results it is concluded that an intrinsic hormone, apparently the adipokinetic hormone, is released from the glandular lobe and regulates the substrate supply from the fat body during flight. It is suggested that the secretory activity of the glandular cells is controlled by the axon terminals on these cells.