Spermducts of H. virescens increased in length and developmental state after injection of 20HE into early pupae. Spermducts culturedin vitrodid not change in the absence of 20HE. incorporated into tissue culture medium induced changes which depended on the amount of time diapausing pupae maintained at 10°C were exposed to warmth and light. Spermducts from diapausing pupae did not respond to 20HE, and spermducts from pupae warmed for two days at LD increased in length but lost visible structure. Spermducts from pupae warmed for four days at LD differentiated to myogenically contracting organs resembling the seminal vesicles of adult moths. Thus, the degree to which post-diapause spermducts developedin vitroin the presence of 20HE depended on the developmental age of the donor.