Light deprivation by blinding leads to involution of the reproductive organs and cessation of cyclic vaginal phenomena in adult fe-male hamsters. Associated with the atrophy of the reproductive system are elevated LH levels and depressed prolactin levels in the anterior pituitary gland. Chronic treatment (15 or 30 days) of blind hamsters with synthetic luteinizing hormone releasing factor (LRF) decreased pituitary levels and elevated plasma titers of LH. Neither pituitary nor plasma levels of prolactin were influenced by LRF treatment. Pinealectomy completely reversed the effects of blinding on the reproductive organs and on the pituitary levels of LH and prolactin.