Abstract. The influence of different photoperiods on the secretion of metatonin and N-acetylserotonin (NAS) in the female rabbit was investigated. Under all photoperiodic conditions, 12-h light: 12-h dark (12L:12D), 2L:22D and 22L:2D, mean melatonin levels were higher during the scotophase. By contrast, mean NAS levels were 2–6-fold greater in the photophase compared to levels in the scotophase. This dissociation of circulating melatonin and NAS suggests that in the rabbit melatonin secretion is tightly controlled by the scotoperiod and NAS is probably produced by and originates from other organs as well as the pineal.