A study of the effect of continuous light on the weight of the pineal organ in rats following gonadectomy, hypophysectomy or adrenalectomy, and in animals fed thiouracil, was undertaken in an effort to determine whether or not the reduction in pineal weight induced by light is hormone-dependent. Immediately after the removal of their gonads, adrenals or pituitaries, or with the institution of a thiouracil diet, 307 22-day-old male and/or female rats of the Sprague-Dawley strain were placed in continuous light or under usual day-night conditions. Nine to 10 weeks later, body and pineal weights were taken, as well as thyroid weights in the thiouracil series. The weight of the pineal body was found to be reduced significantly in all animals housed in continuous light, indicating that this change is mediated through the central nervous system and is not dependent upon secretions of the pituitary, the gonads, the adrenals or the thyroid. A slight increase in the pineal weight of gonadectomized animals under usual day-night conditions and an equal hypertrophy of the thyroid in rats fed thiouracil, irrespective of their exposure to light, were also observed. The response of the pineal organ to gonadectomy suggests that a negative feedback system exists between the pituitary and the pineal. The similarity in the degree of thyroid hypertrophy in thiouracil-fed animals reflects an equal release of the thyroid-stimulating hormone in rats exposed to continuous light or housed under usual laboratory conditions.