We have studied the effect of aqueous extract of raw garlic and boiled garlic on cyclooxygenase activity in rabbit tissues. Raw garlic inhibited cyclooxygenase activity non-competitively and irreversibly. A dose-dependent inhibition of cyclooxygenase activity was observed in tissues treated with raw garlic. The garlic concentrations required for 50% inhibition of platelets, lung and vascular aortic cyclooxygenase activities of rabbits were 0.35, 1.10 and 0.90 mg, respectively. Cyclooxygenase activity of rabbit platelets was more sensitive to inhibition by raw garlic than the enzyme from blood vessels or lungs. Boiled garlic was found to have little effect on cyclooxygenase activity as compared to raw garlic in these tissues. This may be because the active component of raw garlic is destroyed upon heating. These results indicate that garlic may be beneficial in the prevention of thrombosis if ingested raw rather in a cooked form.