Daily 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 seasonality.