Although the state of hyperhomocysteinemia (HHcy) appears to be associated with higher risks of coronary, cerebral and peripheral vascular disease as well as with a number of other clinical conditions, the underlying molecular mechanisms are not fully elucidated. There is strong evidence, however, that HHcy could induce a pathogenic state of oxidative stress. The interest in modulating the elevated levels of total homocysteine in HHcy and/or their negative impacts through preventive strategies, particularly through the supplementation with vitamins that may be linked to the homeostasis of homocysteine (folate, vitamin B(12), and vitamin B(6)), has increased in recent years. Here we show that active antioxidant components of the traditionally used black seeds of Nigella sativa plant protect against the development of methionine-induced HHcy and its associated state of oxidative stress. Pretreatment of rats with an oral dose of 100 mg/kg of thymoquinone, the main active constituent of the black seed, for 30 min and for 1 week almost completely protected against induced HHcy measured 5 h after methionine load (100 mg/kg). Under similar conditions pretreatment with commercial black seed oil (100 microl/kg) for 30 min and for 1 week produced significant and strong protection levels of 74.2 and 94.5%, respectively. Under the state of induced HHcy there were significant increases in the plasma levels of triglycerides, lipid peroxidation, cholesterol and in the activities of glutathione peroxidase and superoxide dismutase. Catalase activity was not affected. The total antioxidant status, however, was significantly depressed. All of these effects were almost totally blocked by prior treatment with thymoquinone or black seed oil. These findings may contribute towards a protective measure utilizing the black seed against the negative impacts of HHcy.