Abstract.Face flies overwinter as adults in reproductive diapause. Administration of 20‐hyroxyecdysone and/or methoprene induced reproductive development in diapausing flies which were maintained in a diapause‐inducing environment. Hormone effects were additive and female flies were more sensitive than males. Release of vitellogenin from cultured fat body was stimulated by 20‐hydroxyecdysone or methoprene. Transfer of flies from diapause to diapause‐breaking environments induced some to break diapause, but this decreased with the time flies had been in a diapause‐inducing environment. In contrast, topical application of methoprene to diapausing flies induced reproductive development irrespective of their ages even when they were kept in the diapause‐inducing environment for 80 day degrees above a 12°C base temperature (14.5 days). Therefore diapause induction must depend on hormone levels less than some threshold level. The putative threshold varied according to diapause propensities of different genetic lines. Lines showing high frequencies of diapause required greater amounts of methoprene for reproductive development in diapause conditions than did lines showing low frequencies