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Two experiments were conducted to test the influence of ambient light and the pineal gland on the pituitary-thyroidal and pituitary-gonadal axes of female golden hamsters. The results show that the thyroid glands of dark-exposed 1light:dark (LD) 1:23 in hours], goitrogen-treated (1% aqueous thiourea as drinking fluid) animals hypertrophy less than those of similarly treated animals maintained in long daily periods of illumination (LD 16:18). Goitrogen-induced thyroidal hypertrophy is accompanied by hypothyroidism. The hypertrophic response of the thyroid glands in either light cycle was not affected by removal of the pineal gland. Microscopically the thyroid glands of goitrogen-treated animals kept in either long or short daily photoperiod exhibited changes normally associated with hypertrophy and concomitant hypofunction of the gland.The exposure of intact female hamsters to LD cycles of 1:23 caused a significant reduction in the weight of the uteri and adrenal glands within 35 days. Likewise, removal of the eyes led to atrophy of the same organs. Although darkness caused a variable change in ovarian weight, histological studies of ovaries from dark-exposed and blinded animals suggested minimal hormonal output. The atrophic responses of the ovaries, uteri and adrenal glands were prevented by pinealectomy. These data suggest that darkness activates the pineal gland to secrete a substance(s) which modified pituitary-target organ relationships.Hypothyroidism induced by the administration of thiourea to intact female hamsters in both long (LD 16:s) and short (LD 1:23) daily light cycles caused a reduction in weight of the ovaries, uteri and adrenal glands. With one exception, these responses were not altered by pineal removal. The uterine weights of the dark-exposed hypothyroid animals that were pinealectomized were comparable to those of animals given fresh drinking water and maintained in LD cycles of 16:s (normal controls).The relationship of the secretion of thyroid stimulating hormone to adrenocorticotrophic and gonadotrophic hormone release by the pituitary is discussed.We have demonstrated recently that exposure of intact male hamsters (Hoffman, Hester and Towns, '65; Hoffman and Reiter, '65a) to short daily periods of light [light:dark (LD) 1:23 in hours] during the winter months results in atrophy of the reproductive organs. This atrophy is prevented, however, if the animals are first pinealectomized (Hoffman and Reiter, '65a). These data indicate that the influence of (or absence of) light on the reproductive system is mediated via the pineal gland. Since the length of the daily light period influences the thyroidal secretory activity of rats and mice (TixierVidal, '59), we tested the effects of pinealectomy and the length of the daily photoperiod on the thyroidal response of female hamsters to goitrogen treatment. Additional observations were made on the adrenals, ovaries and uteri.J. EXP. ZOOL., 162: 263-268. MATERIALS AND METHODSOne hundred-thirty-two young adult female golden hamsters (Mesocric...