Intrinsic gland cells of the corpus cardiacum of Locusta synthesize AKH, and release it into the haemolymph shortly after onset of flight. AKH stimulates the release of diacylglycerol — the fuel for the flight muscles — from the fat body. The peak of haemolymph AKH is reached after about 30 min flight. Release of AKH mainly occurs from short processes of the gland cells, by means of exocytosis. It may be expected that this release is followed by compensatory re-uptake of membrane material by means of endocytosis near the site of exocytosis. To get information on the location of endocytotic activity and on the intracellular endocytotic pathway, the uptake of the tracer horse-radish peroxidase (HRP) by the gland cells in flying and in resting locusts was studied electron microscopically