Rhythmic melatonin secretion from the medaka pineal organ in flow-through culture (Iigo et al., unpublished data). The medaka pineal organ was dissected out and maintained in flow-through culture in a light–dark cycle for 1 day and constant darkness for 3 days. The medaka pineal organ released melatonin in a rhythmic fashion, releasing a larger amount during the dark phase of the light–dark cycle. This rhythm persisted even under conditions of constant darkness, with an approximately 25-hour period (i.e., a circadian rhythm). Open and closed bars indicate the light and dark phases, respectively.