We have studied the function of the pineal gland as putative endocrine transducer of daily and seasonal photic information in the tropical catfish, Oxydoras sifontesi by measuring plasma melatonin variations. The daily pattern of plasma melatonin concentration was determined in adult catfish kept under natural photoperiod or under constant darkness, blood-sampled over 24-hour periods. To study the annual variations in plasma melatonin, midday and midnight blood samples were obtained four times in one year. Daily and annual profiles of plasma melatonin concentration are discussed in the context of reproductive function and correlated to the annual gonadosomatic index (GSI) variation.