Female rats aged I month and older were kept indoors in darkness, under lighting for 24 hours or exposed to ordinary lighting (light--12 hrs; darkness--12 hrs). Three weeks after the beginning of the experiment, they received N-nitrosomethyl urea (NMU) treatment in the dose of 50 mg/kg, intravenously, at weekly intervals, and were exposed to alternating 50 Hz (AMF) or static 0.2 oersted, (DMF) electromagnetic field radiation for 3 hrs, daily. Under usual lighting conditions, NMU treatment was followed by the development of mammary gland adenocarcinoma (MGA) in 31% of the animals. However, AMF or DMF treatment was followed by a shorter period of latency, without affecting MGA frequency. Constant lighting was found to stimulate a sharp increase in carcinogenesis: MGA frequency in groups receiving NMU, NMU+AMF or NMU+DMF was 57, 81 and 61%, respectively; latency period dropped by half as compared with usual lighting conditions. Conversely, the rats constantly kept in darkness revealed significant inhibition of mammary gland carcinogenesis, MGA frequency in NMU, NMU+AMF or NMU+DMF groups being 3, 4 and 2%, respectively. A blood serum--hormone assay showed constant lighting to decrease melatonin level and to increase prolactin concentration for all the modalities of carcinogenic treatment used. The results point to a significant promoting effect of constant lighting on mammary gland carcinogenesis by NMU in rats. This effect is potentiated by additional AMF or DMF treatment. Conversely, constant darkness inhibits mammary gland carcinogenesis and mitigates the pernicious effect of low-frequency electro-magnetic radiation.