Face flies, Musca autumnalis DeGeer (Diptera: Muscidae), overwinter as adults in reproductive diapause. The onset of diapause in face fly populations varies with longitude and is regulated by photoperiod. Therefore, the threshold of diapause response must be under genetic control. The mode of diapause inheritance in face flies was analyzed by crosses between high and low diapause strains. The endocrinology of diapause and reproductive developments was investigated by using in vivo and in vitro hormonal applications to see how juvenile hormone and ecdysteroid levels are regulated;Face fly diapause was polygenic with a small number of genes. Diapause inheritance did not fit a simple additive model because of incomplete dominance and sex-linkage. The incidence of face fly diapause was correlated with genes controlling developmental rates. A progressive decrease of diapause incidence in laboratory colonies was a correlated response to selection for increased developmental rates;Vitellogenin and vitellin of face flies were characterized by SDS-PAGE and Western blotting and quantified in hemolymph from reproductive and diapausing flies by using indirect ELISA. Reproductive females began to synthesize vitellogenin soon after adult eclosion, between 0 and 15 degree-days above a 12°C threshold. Vitellogenin synthesis was sigmoid with respect to time. Pre-diapause, teneral adults also synthesized small amounts of vitellogenin;Methoprene (a juvenile hormone analog) and 20-hydroxyecdysone induced diapausing flies to undergo reproductive development even when maintained in diapause conditions. The effects of both hormones were additive. Newly synthesized vitellogenin was measured in fat body culture medium by indirect ELISA. Fat body from diapausing flies synthesized vitellogenin in vitro in the presence of methoprene or 20-hydroxyecdysone. The foregoing data suggest that both hormones must be maintained at low levels during diapause development.