Synthesis and release of the pineal hormone melatonin is in all vertebrates primarily regulated by the light/dark cycle. In pineal organs of teleost fish, like in other non-mammalian vertebrates, melatonin formation is regulated by a direct photoreception of the pineal organ. We performed measurements in explanted, perifused pineal organs of the rainbow trout, Oncorhynchus mykiss, to examine whether melatonin can influence its own production. For this purpose we have continuously perifused isolated pineal organs under light- and dark-adapted conditions and measured the release of melatonin and other methoxy-indoles by HPLC with electrochemical detection. Addition of 2-iodomelatonin to the perifusate in a concentration of 2 ng/ml significantly inhibited melatonin release in light-, as well as in dark-adapted organs. The release of 5-methoxytryptamine and 5-methoxytryptophol was also significantly reduced in light-adapted organs. These results indicate that extracellular melatonin may act as a paracrine or autocrine feedback signal and may be important for the illumination-dependent melatonin production.