Summary Pupal-adult development in post-diapause pupae of M. configurata is reversibly blocked by high temperature. At 27.5°C only 10% of pupae have blocked development but at 30°C 100% are blocked. The sharpness of the transition indicates that a single, controlling event in development is affected by high temperature rather than multiple developmental events. Pupae fist incubated at 20°C for about three days and then incubated at 30°C develop, demonstrating that the initiation of development is blocked by high temperature rather than metamorphic development per se. An endocrine basis for the developmental block was established by treating pupae held at 30°C with 20-hydroxyecdysone which at low, physiological doses (ED50 = 0.5 nmoles/g live wt) initiated development in a sigmoid, dose-response fashion. Cyclic GMP was also effective in initiating development, presumably by stimulating the production of 20-hydroxyecdysone or interacting synergistically with it. It is concluded that high temperature causes a reversible, stress-like response in pupae of M. configurata in which the endocrine system is suppressed and the initiation of development is blocked.