We investigated the effects of the fixed oil of Nigella sativa seeds in rats by monitoring blood homeostasis and body weight as well as toxicity. Animals were treated daily with an oral dose of 1 ml/kg body weight of the N. sativa seed fixed oil for 12 weeks. Changes in key hepatic enzymes levels were not observed in N. sativa treated rats after 12 weeks of treatment. The serum cholesterol, triglycerides and glucose levels and the count of leukocytes and platelets decreased significantly by 15.5, 22, 16.5, 35 and 32%, compared to control values, respectively; while haematocrit and haemoglobin levels increased significantly by 6.4 and 17.4%, respectively. In parallel, significant slowdown of the body weight evolution was observed in N. sativa treated animals comparatively to the animal control group. On the other hand, no mortality was noted for ten times the therapeutic dose in mice, during 15 days period after the oil administration (10 ml/kg p.o.). These results support the traditional use of N. sativa seeds as a treatment of the dyslipidemia and the hyperglycaemia, and related abnormalities; however, indicate a relative toxicity of this plant. Acute and chronic toxicity, and the mode of the action of the N. sativa fixed oil must be studied.