Diethylnitrosamine-induced hepatic lesions are greater in rats maintained under a light-dark cycle than under constant light, related to the locomotor activity rhythm.

Environmental lighting conditions affect circadian rhythm and carcinogenesis. The effect of diethylnitrosamine (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 in locomotor 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 and LL. Two weeks after DEN injection, locomotor activity in 24 hr had increased under the LD. Circadian rhythmicity might be coupled with the carcinogenicity of DEN.