Pineal photoreceptor cells produce the neurohormone, melatonin, a major "Zeitgeber" of the organism. This compound has been involved in the control of development, growth, sexual maturation, and seasonal reproductive cycles. We investigated, here, the effects of estradiol-17 beta on melatonin production by cultured pineal photoreceptor cells. Under a light/dark (LD: 12/12) cycle, cultured trout pineal cells maintained a rhythmic secretion of melatonin with higher amounts being released during the dark phase. The amplitude of the rhythm tended to increase with time spent in culture. Application of estradiol-17 beta during the dark phase of a LD cycle (i.e., for 12 h) affected melatonin release in a dose-dependent manner: low concentrations (10(-10) to 10(-8) mol/l) were inhibitory whereas high concentrations (over 10(-7) mol/l) were stimulatory. When estradiol-17 beta was applied continuously for several 24 h LD cycles, the inhibitory effect observed during the first dark phase disappeared later on. Rather, in the presence of estradiol-17 beta, at the concentrations of 10(-9) and 10(-6) mol/l, a high amplitude rhythm in melatonin secretion was recovered faster, when compared to controls. Replacement of 10% fetal calf serum by 0.1% bovine serum albumin did not affect the responses of the photoreceptor cells to 10(-6) mol/l of estradiol-17 beta. The present results bring the first evidence that estradiol-17 beta modulates melatonin secretion by cultured pineal photoreceptors. They further support the idea according to which sexual hormones exert a feedback regulation on the pineal. Pineal photoreceptors appear as multieffector cells which transduce information from, both, physical (photoperiod) and internal (chemical) factors.