The in vitro effect of aqueous extract of Nigella sativa seeds on nitric oxide (NO) production by murine macrophages was studied. Murine peritoneal macrophages were pre-incubated with the extract and then activated with Escherichia coli lipopolysaccharride. NO production was measured after 24 hours by spectrophotometry. The plant extract caused a dose-dependent decrease in NO production. Dialyzed preparation of the extract did not affect NO production. However, the boiled fraction of the extract resulted in a dose-dependent inhibition of NO apparently comparable to that of the whole extract. These results indicate that the aqueous extract of N. sativa seeds exhibits an inhibitory effect on nitric oxide production by murine macrophages and the active component(s) is/are non-protein in nature. In view of the fact that nitric oxide is a pro-inflammatory mediator, this study validates the traditional use of the Nigella sativa seeds for the treatment of rheumatism.