We investigated the toxicity of the fixed oil of Nigella sativa L seeds in mice and rats through determination of LD50 values and examination of possible biochemical, hematological and histopathological changes. The acute toxicity of Nigella sativa fixed oil was investigated in mice. LD50 values, obtained by single doses, orally and intraperitoneally administered in mice, were 28.8 ml/kg body wt. p.o. [26.2-31.6] and 2.06 ml/kg body wt. i.p. [1.86-2.26], respectively. Chronic toxicity was studied in rats treated daily with an oral dose of 2 ml/kg body wt. for 12 weeks. Changes in key hepatic enzymes levels, including aspartate-aminotransferase, alanine-aminotranferase, and gamma-glutamyltransferase and histopathological modifications (heart, liver, kidneys and pancreas) were not observed in rats treated with Nigella sativa after 12 weeks of treatment. The serum cholesterol, triglyceride and glucose levels and the count of leukocytes and platelets decreased significantly, compared to control values, while hematocrit and hemoglobin levels increased significantly. A slowing of body weight gain was also observed in Nigella sativa treated rats, as compared to control animals. The low toxicity of Nigella sativa fixed oil, evidenced by high LD50 values, key hepatic enzyme stability and organ integrity, suggests a wide margin of safety for therapeutic doses of Nigella sativa fixed oil, but the changes in hemoglobin metabolism and the fall in leukocyte and platelet count must be taken into consideration.