Using an in vitro prothoracic gland assay (PTG) to assess production of ecdysteroids, a low molecular weight ecdysiotropin(s) (500-1,500 daltons) was detected in proctodaea (hindguts) of unparasitized and parasitized Manduca sexta 5th instars. Dose-response curves for the three groups of insects tested were similar with maximum stimulation being observed in PTGs incubated with 0.00125-0.005 organ equivalent. However, when results were adjusted for differences in wet weight of the proctodaea, extracts from hosts which initially were parasitized in the first instar, possessed 2.6 times more ecdysiotropic activity than extracts from day-3 unparasitized 5th instars and 1.8 times more activity than extracts from day-1 unparasitized 5th instars. These results suggest that the parasite may be able to regulate the synthesis and/or release of its host's proctodaeal ecdysiotropin(s) possibly as a strategy of host regulation to facilitate meeting its own developmental requirements.