: Intrasplenic ovarian tumorigenesis is the most suitable experimental model to study the role of hormones in carcinogenesis. If increased secretion of gonadotrophins causes ovarian tumorigenesis in the spleen, one can find ways to control the secretion of gonadotrophins so as to prevent the tumor formation. Pineal is the one gland which is known to regulate gonadotrophin secretion. In the present studies, isologous pineals were transplanted into the anterior eye chamber of Swiss females bearing autologous ovaries in the spleen, with appropriate controls. The results showed that two pineals maintained in the ocular chamber for four months could prevent tumor formation of the ovarian transplants in the spleen. Control animals maintained for the same period of time as experimental animals had ovarian transplants turn into a tumor. These experiments clearly demonstrated that the pineal had an inhibitory effect on intrasplenic ovarian tumorigenesis. Since two pineals were required to bring about the effect, it implies that the amount of the factor present in the endogenous gland is not enough to prevent the tumorigenesis. The inhibitory action in these experiments must be via the pituitary, since it is not inhibition of tumor growth but it is prevention of transformation of the normal ovary into a tumor. This prevention is possible only if hormone stress is withdrawn from the system. These results suggest the prospects of prevention of cancer by some factor present in the pineal.