The cytological appearance of the pineal body is related to stage of development or age, time of day, exposure to illumination, season of the year, and reproductive state. Experimental manipulations that alter the normal cytology of the pineal include hypophysectomy, administration of any of several hormones or drugs, blinding, denervation and exposure to stressful influences such as heat or cold. The effects of blinding are similar to the effects of continuous darkness on the pineal's cytology. Ultrastructural examinations reveal a number of changes in the organelles, especially an increased number of vesicles and complex membranous whorls. These features suggest heightened activity and generally are coincident with sexual quiescence. Membranous whorls also may be induced by cold exposure. Prolonged or continuous illumination leads to a reduction in the size of pinealocytes and to changes in their organelles that suggest reduced activity. Such alterations can often be correlated with heightened sexual activity. Sympathetic denervation of pineal glands also results in atrophy of the pinealocytes. Reproductive condition is reflected in the appearance of the pinealocytes. The estrous cycle, pregnancy, castration and the administration of gonadotropins or gonadal steroids affect pinealocyte structure. A feedback system involving the gonads and pineal gland is indicated.