1. Trilinolein, a triacylglycerol with linoleic acid as the only fatty acid residue in all three esterified positions of glycerol, was recently reported to have an inhibitory effect on adrenaline-induced platelet aggregation. In the present study, we found that trilinolein at concentrations ranging from 0.01 to 1 microM increased cyclic GMP formation and decreased cyclic AMP formation in washed human platelets. Both NG-nitro-L-arginine methyl ester (L-NAME) and methylene blue attenuated the trilinolein-induced increase in cyclic GMP. 2. Adrenaline decreased not only the production of cyclic AMP but also that of cyclic GMP. Trilinolein antagonized the inhibitory effect of adrenaline on cyclic GMP formation, but potentiated the inhibitory effect of adrenaline on cyclic AMP accumulation. 3. Both trilinolein and adrenaline enhanced intracellular calcium but the increment of intracellular calcium induced by them was much less than that produced by thrombin. 4. We propose that the anti-platelet effect of trilinolein is mediated through an increase in cyclic GMP, and that the change in cyclic GMP results from stimulation of nitric oxide synthesis in platelets. 5. We also propose that reduction of both cyclic AMP and cyclic GMP are involved in adrenaline-induced platelet aggregation.