The modulating effect of thymoquinone (TQ) on benzo(a)pyrene (BP)-induced forestomach tumours was investigated in female Swiss albino mice, receiving oral administration of BP at a dose of 1 mg twice weekly for 4 weeks. Administration of 0.01% of TQ in drinking water 1 week before, during and after BP treatment until the end of the experiment resulted in significant suppression of BP-induced tumourigenesis when compared with the group receiving BP alone. TQ inhibited both BP-induced forestomach tumour incidence and multiplicity by 70% and 67%, respectively. Lipid peroxide accumulation and decreased glutathione (GSH) content and glutathione-S-transferase (GST) and DT diaphorase activities were observed in the liver of BP-treated tumour-bearing mice. TQ alone showed a significant induction in the enzyme activities of hepatic GST and DT diaphorase. Mice treated with TQ along with BP showed almost normal hepatic lipid peroxides and GSH levels, and normal enzyme activities compared to the control group. The present data may indicate the potential of TQ, the main constituent of the volatile oil of Nigella sativa seed, as a powerful chemopreventive agent against BP-induced forestomach tumours in mice. The possible modes of action of TQ may be through its antioxidant and anti-inflammatory activities, coupled with enhancement of detoxification processes.