Samples of the expressed fixed oil from different sources of Nigella sativa seeds were examined by thin-layer and gas chromatography for content of fixed oils and thymoquinone, and these substances were tested as possible inhibitors of eicosanoid generation and membrane lipid peroxidation. The crude fixed oil and pure thymoquinone both inhibited the cyclooxygenase and 5-lipoxygenase pathways of arachidonate metabolism in rat peritoneal leukocytes stimulated with calcium ionophore A23187, as shown by dose-dependent inhibition of thromboxane B2 and leukotriene B4, respectively. Thymoquinone was very potent, with approximate IC50 values against 5-lipoxygenase and cyclo-oxygenase of < 1 microgram/ml and 3.5 micrograms/ml, respectively. Both substances also inhibited non-enzymatic peroxidation in ox brain phospholipid liposomes, but thymoquinone was about ten times more potent. However, the inhibition of eicosanoid generation and lipid peroxidation by the fixed oil of N. sativa is greater than is expected from its content of thymoquinone (ca. 0.2% w/v), and it is possible that other components such as the unusual C20:2 unsaturated fatty acids may contribute also to its anti-eicosanoid and antioxidant activity. These pharmacological properties of the oil support the traditional use of N. sativa and its derived products as a treatment for rheumatism and related inflammatory diseases.