[Susceptibility of rats to benzene during morning and evening]

For several chemicals time-related differences in the intensity of the toxic effects in animals were observed. Partially these phenomena are a result of changing capability of the liver and other organs to detoxify. The aim of the experiment was to evaluate the sensitivity of rats to benzene in the morning and in the evening, and to test whether benzene hydroxylase activity in liver varies in circadian manner. The experiment was carried out on 110 rats, some of them maintained under standard LD conditions, some exposed to continuous darkness or continuous illumination. It was found that sensitivity of rats subacutely poisoned with benzene, expressed by reduction of lymphocyte count in peripheral blood, was significantly higher in the evening than in the morning at standard LD conditions. Benzene hydroxylase activity in the liver microsomes was highest at 22.00. Continuous darkness or continuous illumination modified both effects. The results indicate that the sensitivity of rats to benzene varies with the time of administration of this compound. It may be related to daily variations in activity of microsomal enzymes which metabolise most of the xenobiotics.