The eclosion hormone triggers a stereotyped preprogrammed pattern of behavior in silk moths. The effects of the hormone were duplicated by the injection of dibutyryl adenosine 3′,5′-monophosphate, adenosine 3′,5′-monophosphate (cyclic AMP), or guanosine 3′,5′- monophosphate (cyclic GMP) into theophylline-treated pharate moths. Treatment with theophylline reduced the latency of the response to a low dose of hormone, presumably by blocking phosphodiesterase. Endogenous levels of cyclic AMP, but not cyclic GMP, increased significantly in the central nervous system within 10 minutes after hormone injection. We conclude that an early step leading to the release of the eclosion motor program is an increase in cyclic AMP in target neurons of the central nervous system.