The role of the pineal gland in inducing and maintaining the persistent estrus of rats exposed to continuous illumination was examined. Under the cyclic illumination, the pinealectomy or sham-operation revealed no effect on the estrous cycle, although weights of the ovaries, adrenals and hypophysis were slightly but definitely greater in the pinealectomized animals. In rats exposed to the continuous illumination immediately after pinealectomy or sham-operation, both groups exhibited the persistent estrous states soon after the change of lighting condition. In these rats, neither the degree of persistent estrus nor the organ weights at autopsy showed any significant difference between the groups. Moreover, the pinealectomy could not alter the incidence of estrous phases in persistent estrous rats which had been established already by the continuous illumination.