Octopamine and an agonist, chlordimeform, increase the responsiveness of adult and pharate adult Manduca sexta to gentle mechanical stimulation of the wing. Higher doses of chlordimeform elicit almost continuous production of the flight motor pattern in both adults and pharate adults, and the effect persists for more than 24 h. The dose of chlordimeform necessary for this effect increases with age. Mechanical stimulation of the wing of pharate adults elicits several cycles of flight motor pattern, but with repeated stimulation the animal habituates. Habituation is slower in chlordimeform-treated animals than in controls. Injection of octopamine (1-8 X 10(-8) mol) or chlordimeform (3 X 10(-9) mol) into the mesothoracic ganglion elicits the flight motor pattern. The excitatory actions of both compounds can be blocked by cyproheptidine. Chlordimeform (5 X 10(-8) mol) in acetone applied to the wing does not cause a noticeably greater increase in teh electrical activity of sensory neurons than does acetone applied alone; this result suggests that chlordimeform does not act on these peripheral sites or on axonal membranes in general. We suggest that chlordimeform and octopamine act on the thoracic ganglia to alter the level of excitation or effectiveness of synaptic transmission among central neurons, including those involved in producing the flight motor pattern.