Mummichogs (Fundulus heteroclitus) were exposed to various regimens of temperature and photoperiod in order to develop a routine husbandry procedure so as to have throughout the year a population of reproductively healthy fish with a good quantity and quality of prematuration follicles (1.2-1.5 mm) suitable for carrying out homologous bioassays ofF. heteroclitus gonadotropin. During the fall and winter months, wild fish in the field all had regressed ovaries (gonadosomatic index <1). On the other hand, laboratory-maintained fish, with or without cold temperature (15°C) and short photoperiod (10 h light/day) pretreatment, generally had sexually mature ovaries (GSI>10) when maintained on a warm temperature (25°C) and long photoperiod (14 h light) protocol. Ovarian follicles retrieved from laboratory fish were responsive toF. heteroclitus pituitary extract stimulation, and underwent germinal vesicle breakdown normallyin vitro. Hence these ovarian follicles served well as a bioassay forF. heteroclitus gonadotropin even outside of the normal breeding season. The pituitary glands retrieved from laboratory fish in winter also retained high gonadotropic potencies, in terms of maturational and steroidogenic activities. Our results thus demonstrated that active gametogenesis in laboratory-maintained fish can be extended five months beyond the end of the normal breeding season. Apparently,F. heteroclitus in Florida is potentially a continuous breeder when under favorable conditions, but has a reproductive quiescent period imposed upon it by some environmental stressor(s). Although the design of the present experiments did not determine the relative importance of nutritional factors, temperature, and photoperiod on the annual reproductive cycle ofF. heteroclitus, there are indications that diatary factors may play a much more dominant role in the reproductive cycle than previously recognized.