Acute toxicity of benzene in rats at different periods of day and in different light/dark regime.

Acute toxicity of benzene expressed by survival time and mortality percentage of rats varied in circadian manner. Survival time of rats maintained under standard light/dark (L/D) conditions was significantly shorter when benzene was injected at 20.00 or 24.00, than at 08.00. Simultaneously, mortality of rats was reversed. Inversion of L/D regime of exposure to continuous light abolished all those effects. Rats exposed to continuous darkness showed significantly longer survival time when benzene was administered at 12.00 or 24.00 than when it was injected at 04.00. Generally, acute toxicity of benzene in rats exposed to light was markedly less pronounced than in animals exposed to darkness.