AbstractDaily injections of dihydroxyphenylalanine (DOPA), a dopamine precursor, stimulate large increases in reproductive and body fat indices in gulf killifish,Fundulus grandis, when given 20 hours after daily injections of 5‐hydroxytryptophan (5‐HTP), a serotonin precursor. However, DOPA injections are inhibitory when given 8 hours after 5‐HTP. The drug effects persist for at least 3 weeks following drug treatment indicating a possible resetting of an endogenous mechanism involved in season