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1. The effects of the volatile oil (V.O.) of the black seed (Nigella sativa) on the arterial blood pressure and heart of urethane-anaesthetized rats were investigated and the effects were compared with those of its constituent thymoquinone (T.Q.). 2. Intravenous administration of V.O. in the dose range (4-32 microliters kg.-1) or T.Q. (0.2-1.6 mg kg-1) to rats decreased the arterial blood pressure and the heart rate in a dose-dependent manner. 3. The effects of V.O. were significantly antagonized by treatment of the animals with cyproheptadine, hexamethonium atropine and by spinal pithing. 4. Treatment of the animals with reserpine (5 mg kg- 1 day-1 for 2 days) significantly antagonized the cardiovascular depressant effects induced by 4 and 8 microliters of V.O. kg-1 but not those induced by the larger doses. 5. T.Q.-induced cardiovascular depressant effects were significantly antagonized by atropine and cyproheptadine but not by reserpine. 6. The results suggested that V.O.-induced cardiovascular depressant effects were mediated mainly centrally via indirect and direct mechanisms that involved both 5-hydroxytryptaminergic and muscarinic mechanisms. The direct mechanisms may be due to the presence of T.Q. in the V.O. The V.O. seemed to possess the potential of being a potent centrally acting antihypertensive agent.