The formation of radiolabelled oxygenated products of arachidonic acid in thrombin-stimulated, [3H]arachidonic acid-prelabelled human platelets is inhibited in a concentration-dependent manner by BW 755C (3-amino-1-[m-(trifluoromethyl)phenyl]-2-pyrazoline) or propyl gallate, both of which are combined inhibitors of lipoxygenase and cyclooxygenase. These compounds do not inhibit the thrombin-induced decrease in the radioactivity of platelet phospholipids but, instead, allow the accumulation of free radiolabelled arachidonic acid. Thrombin causes an increase in the levels of free, endogenous palmitic, stearic, oleic, linoleic and arachidonic acids of up to 10 nmol/10(9) platelets. In the presence of BW 755C or propyl gallate, further increases in the level of free arachidonic acid, of 20-50 nmol/10(9) platelets, occur. The enzyme inhibitors do not affect the accumulation of the other free fatty acids. The increase in arachidonic acid is optimal at 1 U/ml thrombin and 60% complete by 1 min at 37 degrees C. In the platelets from eight donors, the average increases in free fatty acids (in nmol/10(9) platelets) induced by 5 U/ml thrombin in 5 min at 37 degrees C in the presence of 100 microM BW 755C were 1 for linoleic acid, 3.6 for oleic acid, 4.5 for palmitic acid, 7.6 for stearic acid and 32.0 for arachidonic acid.