The modified photoreceptor cell (MP), homologous to the cone-like photoreceptor, is the principal cell type of the avian pineal organ. Two types of secretions (indoles and specific proteins-- pinealins ) are produced in this polarized cell. The synthesis of indoles undergoes striking circadian and circannual changes. MP appear to be receptoneuroendocrine cells which transduce circadian information into 5- methoxyindolic (melatonin, 5-methoxytryptophol, 5-methoxytryptamine) and possibly peptidic neurohormones. Taking into account different stages of differentiation and/or different physiological states (due to cyclic changes) for MP, several mechanisms of transduction of photoperiodic inputs into circadian hormonal outputs have been hypothesized.