The face fly, Musca autumnalis, exposed to shifts of an LD16:8 lighting schedule at varying intervals, whether previously untreated or given placebo or ACTH 1-17 treatment, before the initiation of shifts, exhibits an infradian frequency response in mortality. At overall 50% mortality, a periodicity of approximately 4.5 days is found for flies exposed to placebo or ACTH 1-17 as a response to the shift interval. As compared to controls, the mortality of flies treated with placebo or ACTH 1-17 is delayed. Not all shift schedules are detrimental; some are actually beneficial.