Pinealectomy had inconstant effects on serum gonadotropins when blood samples were taken during the day, however late nocturnal titers of both FSH and LH tended to decline and this decline tended to be reversed by pinealectomy. Either anosmia or blinding elevated nocturnal FSH and had a tendency to elevate nocturnal LH titers – effects which were only inconstantly modified by pinealectomy. Blinding plus anosmia lowered both daytime and nocturnal FSH titers; the lowering of nocturnal levels was reversed by pinealectomy. There was a less pronounced lowering of nocturnal LH titers in these doubly-operated animals and again this was partially reversed by pinealectomy. These changes in gonadotropin titers probably accounted at least in part for the testicular atrophy seen in the doubly-operated animals and its reversal by pinealectomy. Following castration, plasma gonadotropin titers rose in all groups and the final level of FSH attained was highest in the pinealectomized animals. Similarly, final levels of FSH and LH were further increased in castrates by blinding, anosmia or blinding plus anosmia, but these changes were not consistently modified by pinealectomy. At sacrifice of the castrates, pituitary FSH and LH concentrations were not modified by pinealectomy, however they were elevated by anosmia or by the double sensory-deprivation. The atrophy of the seminal vesicles induced by castration was partially prevented by pinealectomy, which suggests that the pineal also exerts a direct inhibitory effect on this accessory organ. Alterations in anterior and posterior pituitary weight were also observe