Nigella sativa has been traditionally used for the treatment of inflammations, liver disorders, and arthritis. Experimentally, it has been demonstrated that N. sativa extracts and the main constituent of their volatile oil, thymoquinone, possess antioxidant, anti-inflammatory and hepato-protective properties. To further evaluate the toxicological properties in a metabolically competent cellular system, thymoquinone was applied to primary rat hepatocyte cultures, and both cyto- and genotoxic effects were tested. Mitotic indices and the rates of apoptoses and necroses were determined as endpoints of cytotoxicity, while chromosomal aberrations and micronucleated cells served as endpoints of genotoxicity. In this approach thymoquinone demonstrated cyto- and genotoxic effects in a concentration dependent manner: it induced significant anti-proliferative effects at 20 microM and acute cytotoxicity at higher concentrations. Thymoquinone significantly increased the rates of necrotic cells at concentrations between 2.5 and 20 microM. Furthermore, it induced significant genotoxicity at concentrations > or =1.25 microM. These observations support the previous finding that thymoquinone causes glutathione depletion and liver damage, but contradict the reports indicating antioxidant and anti-clastogenic effects. Thymoquinone might be metabolised to reactive species and increase oxidative stress, which contributes to the depletion of antioxidant enzymes and damage to DNA in hepatocytes treated with high thymoquinone concentrations.