Platelet-activating factor (PAF), a lipid released as a result of immediate allergic reactions from basophils and mast cells as well as by a variety of other cell types and stimuli, is one of the most potent platelet agonists and hypotensive agents known. Equine platelets stimulated over a wide range of PAF concentrations aggregated in a time- and dose-dependent manner. Maximum aggregation was observed at concentrations of PAF as low as 3.58 x 10(-14) M with platelet-rich plasma (PRP) and 3.58 x 10(-16) M with washed platelets. Furthermore, the aggregation observed did not appear to be breed-dependent. Finally, the platelet arachidonate pathway appeared to play no role in PAF-induced aggregation as exogenous arachidonate did not enhance the reaction, nor were equine platelets pretreated with 38 microM aspirin inhibited in their response to PAF. This level of aspirin totally inhibited the equine platelet aggregation response to arachidonate.