Environmental lighting conditions affect circadian rhythm and carcinogenesis. The effect ofdiethylnitrosamine (DEN, i.p., 200 mg/kg) on carcinogenesis and circadian rhythmicity under a light-dark (LD)cycle, constant dark (DD) and constant light (LL) was analyzed in rats. After the recognition of entrainment inlocomotor activity rhythm to LD cycle, animals remained under the LD cycle or were released into DD or LL.Liver carcinogenicity, measured by GST-P immunostaining, was higher under the LD cycle than under DD andLL. Two weeks after DEN injection, locomotor activity in 24 hr had increased under the LD. Circadianrhythmicity might be coupled with the carcinogenicity of DEN.