METHODS This study was designed to compare the effect of suppression of melatonin secretion by bright light in chickens with that of surgical pinealectomy. OBJECTIVE To determine whether suppression of melatonin secretion without surgery in chickens can result in scoliosis development. BACKGROUND Pinealectomy in chickens consistently produces scoliosis with anatomic characteristics similar to those of human idiopathic scoliosis. Conversely, cutting of the pineal stalk without removal of the pineal gland will also result in scoliosis. This study addresses the question of whether constant bright light can induce scoliosis formation, because it is well known that 24-hour bright lighting conditions can suppress the secretion of melatonin to an equivalent level as pinealectomy. METHODS Seventy-seven newborn Nihon chickens were separated into three groups. A control group (n = 21) with no surgery performed; a pinealectomy group (n = 15) that served as surgical controls; and a constant light group (n = 41). The first two groups were kept together in a strict 12-hour light-dark cycle, whereas the third group was separately kept with constant lighting conditions (>100 lux). All the chickens were radiographed at two weekly intervals, and blood was taken during the middle of the light and dark cycles for serum melatonin assay using ELISA. RESULTS Fifty-four percent of the pinealectomized chickens had scoliosis develop by 6 weeks. None of the constant-light chickens or controls had scoliosis develop for up to 11 weeks. Measurements of serum melatonin levels of the constant light group confirm that secretion is suppressed. CONCLUSIONS This study suggests that for scoliosis to develop in chickens, the surgical operation itself is important and challenges the role of melatonin as an isolated etiological factor in the development of scoliosis.