There is increasing evidence that the pineal gland has a role in the control of neoplastic processes. Kerenyi found hyperplasia of the pineal gland in patients with widely disseminated malignant melanoma. Feuer and Kerenyi reported significantly higher serum melatonin levels with malignant melanoma. Beral et al. found 2.1 times increased risks of melanoma and Pasternak et al. described a relative risk of 1.87, related to exposure to fluorescent light. In our study, white New Zealand rabbits were exposed for 12 months to fluorescent light. The serum melatonin levels of the control group were significantly higher, p less than 0.005. According to Cohen, the pineal gland has a role in the etiology of breast cancer. Melatonin inhibits MCF-7 human breast cancer cell growth in culture. In our study of 500 surgically removed breast tumors, 254 were melatonin receptor positive, 246 negative. Most melatonin receptor positive breast cancer occurred between 60-65 years of age, receptor negative breast cancer peaked between 40-45 years. Benign breast tumors were almost invariably melatonin receptor positive. It is proposed that melatonin may have a direct effect on breast tumors, the melatonin receptors being the probable sites of interaction between melatonin and the tumor cell.