We studied the effect of a 4-week intragastric gavage with a petroleum ether extract of Nigella sativa seeds on blood glucose, insulin and lipids in the normal rat. Petroleum ether extract caused a 25% reduction in food intake that translated into a transient weight loss. No sign of toxicity of the plant could be seen in vivo or in vitro. Fasting plasma glucose remained stable throughout Nigella sativa treatment. At the end of the 4-week treatment, Nigella sativa-treated rats had lower fasting plasma levels of insulin and triglycerides, and higher HDL-cholesterol as compared to pair-fed controls. Response to insulin was evaluated in hepatocytes isolated from animals of all groups by Western blot analysis of phosphorylated MAPK p44/42erk and PKB. In vivo Nigella sativa treatment resulted in greater dose-dependent activation of MAPK and PKB in response to insulin. These results suggest that the petroleum ether extract of Nigella sativa has a slight anorexic effect, and that it contains the hypolipidemic activity previously obtained with the plant. More significantly, our data demonstrate that in vivo treatment with the petroleum ether extract exerts an insulin-sensitizing action by enhancing the activity of the two major intracellular signal transduction pathways of the hormone's receptor.