In the present study, we explored the effects of a clonidine-displacing substance (CDS) which was isolated and partially purified from bovine brain. The low molecular weight brain substance competes with clonidine and rauwolscine in rat brain membranes, and mimics clonidine's inhibitory action in rat vas deferens. We find that CDS competes with [3H]rauwolscine-labeled alpha 2-adrenoceptors in human platelets. Further characterization of CDS in human platelets reveals that, like clonidine, it inhibits the epinephrine-induced aggregation, potentiates the ADP- and the collagen-induced aggregation however, by itself, CDS is unable to induce aggregation. Unlike clonidine, CDS does not affect the prostacyclin (PGI2)-stimulated cAMP accumulation in intact platelets. The presence of CDS in human plasma, as we have recently shown, implies a possible role of CDS in the regulation of platelet action.