The gulf killifish Fundulus grandis undergoes an annual cycle of reproductive, metabolic, and behavioral changes. The seasonal changes are determined primarily by water temperature and by endogenous seasonal alterations in the interpretation of water temperature. The evidence suggests that both exogenous and endogenous determinants involve an interaction of two circadian neuroendocrine oscillations. One neuroendocrine oscillation, and its circadian expressions, is entrained by the daily light-dark cycle. The other oscillation, and its circadian expressions, is coupled to the first oscillation in various phase relations as a function of season and ambient temperature. A temporal synergism of these oscillations and their circadian expressions produces appropriate complexes of physiological and behavioral conditions. The circadian rhythms of plasma cortisol and prolactin concentrations appear to be circadian expressions of the two oscillations, and timed daily injections of cortisol and prolactin can produce different reproductive, metabolic, and behavioral conditions as functions of the time relations between the hormone injections. Similarly, daily injections of drugs that influence serotonergic (5-hydroxytryptophan, 5-HTP) and dopaminergic (dihydroxyphenylalanine, DOPA) activities can induce conditions appropriate to either summer or winter and can determine the manner in which a fish reacts reproductively and metabolically to water temperature. Thus, it is proposed that cortisol and 5-HTP injections entrain one of the neuroendocrine oscillations and that prolactin and DOPA entrain the other.