It is generally agreed that arachidonic acid (20:4 ω6) can stimulate platelet aggregation after conversion to prostaglandin G 2 and H 2 and thence to thromboxane A 2. This action is prevented by cyclooxygenase inhibitors. Washed platelets were isolated on metrizamide gradient and resuspended in a Ca 2+-free buffer. Their stimulation by C 20:4 6 was followed by 14C serotonin (5HT) release, thromboxane (TX) synthesis and an increase of light transmission, not dependent on aggregation, accompanied by slight lysis (14%). The addition of extrinsic Ca 2+ suppressed lysis and allowed the formation of aggregates. Under these conditions, cyclooxygenase inhibitors such as acetyl salicylic acid, indomethacin or flurbiprofen totally suppressed TX synthesis without preventing platelet aggregation or [ 14C]-5HT release. Other C 20 polyunsaturated fatty acids could not substituted for C 20:4 ω6 in inducing aggregation, and Ca 2+ was found to be a prerequesite for protection of the cell against lysis as well as for aggregation in the absence or TX formation. The use fo the lipoxygenase inhibitor BW 755 C did not prevent C 20:4 ω6-induced aggregation of aspirin-treated platelets, suggesting that the phenomenon was independent of this pathway also. The total suppression of oxidative metabolism with these inhibitors was verified by the analysis of icosanoids using glass capillary column gas chromatography. It is suggested that under these condition, C 20:4 ω6-induced platelet aggregation might be due to an increased membrane permeability to Ca 2+ induced by this fatty acid in the absence of oxidation.