The involvement of both juvenile hormone acid (JHA) and the sex accessory glands (SAGs) in the reproduction of the male moth Agrotis ipsilon was studied as a function of age and mating status. Total protein content analysis followed by gel electrophoresis of the SAGs, radiochemical assay for JHA biosynthesis and surgical and behavioural experiments were performed. Both the protein content of the SAGs and the biosynthetic activity of the corpora allata (CA) increased with age. Allatectomy and JHA/JH treatments showed that the protein content of the SAGs is linked with the activity of the CA. The protein content of the glands, but not the rate of JHA biosynthesis, decreased just after mating, and both increased sharply 24 h later. Injection of fluvastatin, an inhibitor of JH biosynthesis, in males immediately after mating prevented the increase in JHA synthesis and lowered the total protein content of the SAGs. Moreover, fluvastatin disrupted normal spermatophore transfer during the next mating of the injected males. Our results show that JHA controls the reproduction of A. ipsilon males by its separate actions on the sex accessory glands and on sexual behaviour.