Endoperoxide analogs at doses 100-fold higher than those required to aggregate human platelet-rich plasma (PRP), were ineffective on rat PRP. Indomethacin (20 microM) and imidazole did not affect arachidonate-induced aggregation of rat PRP. On the other hand, prostaglandin (PG) E1 inhibited aggregation at doses similar to those effective on human PRP, while high doses of PGI2 failed to inhibit arachidonate aggregation in the rat. Eicosatetraynoic acid (10 microgram) blocked the second phase of aggregation; the Ca2+-ionophore A 23187 potentiated the arachidonate effect. Thus, it appears that endoperoxides or thromboxane A2 may not be involved in rat platelet aggregation and that the formation of aggregants from arachidonate shares many properties with the biosynthesis of slow reacting substance, a metabolite of arachidonic acid containing a sulphate group. To test this, rat PRP was incubated with labeled sulphate and aggregated with arachidonate. After column and thin layer chromatography a labeled lipid was identified having a mobility higher than phospholipids but lower than PGF2 alpha. Treatment with arylsulphatase decreased radioactivity by at least 70%.