Estrogen effects on pineal secretion of melatonin are controversial. Some feel that estrogen inhibits melatonin output in vitro but not in vivo. Melatonin levels vary with the age in chickens where circulating estrogen levels also vary. Laying hens have minimal melatonin levels and maximal serum iron concentrations. Thus, we reasoned that iron released by estrogen may inhibit melatonin secretion from the chick pineal gland. The present study shows that perifusion of estrogen-treated chick pineal glands with several concentrations of iron greatly inhibited melatonin secretion.