The influence of flight activity on the release of secretory granules from the adipokinetic cells in the corpus cardiacum of Locusta migratoria was studied. Two labeling methods, an enzymatical and a radioactive one, were used to label young, newly synthesized secretory granules and so distinguish them from older, preexisting granules. Both methods demonstrated that the ratio between the numbers of labeled and unlabeled secretory granules was lower in flight-stimulated adipokinetic cells than in unstimulated cells. This ratio was lower in both the cell bodies and the cell processes of flight-stimulated cells. After flight there was no detectable change in the total number of secretory granules, which indicates that the synthesis of new secretory granules is not inhibited by flight activity. Rather, the tendency of flight-stimulated cells to have more trans-Golgi networks labeled with wheat-germ agglutinin-conjugated horseradish peroxidase suggests that the synthesis of new secretory granules was enhanced by flight. The results led to the conclusion that young secretory granules were preferentially released over older secretory granules.