The action of a formamidine pesticide on the ultrastructure of hormone release from the glandular lobe of the corpus cardiacum (CC) of the locust, Locusta migratoria (L.), has been studied. Treatment of the isolated CC with 5 μM desmethylchlordimeform (DCDM) caused depletion of neurosecretory material from the intrinsic cells of the glandular lobe without affecting the nervus corpus cardiacum II axons. Pretreatment of isolated CC with the α-aminergic antagonist (5 μM) phentolamine blocked DCDM-induced release of neurosecretory material from the glandular cells. DCDM-induced release of neurosecretion from the glandular cell axons occurred via exocytosis. DCDM treatment also affected the distribution of mitochondria. Mitochondria from adjacent cells lined up along the plasma membrane, forming pairs in juxtaposition.