The platelet rich plasma (PRP) from most dogs aggregates in response to ADP, collagen and thrombin. Dog PRP generally does not aggregate in response to epinephrine, and previous studies found that dog PRP uniformly failed to aggregate when exposed to arachidonic acid prepared in an ethanol-sodium carbonate medium. Our studies demonstrate that 30% of randomly selected mongrel dogs have PRP which aggregates when exposed to sodium arachidonate dissolved in modified Tyrode's buffer, PRP from these dogs also aggregates with lower concentrations of ADP and collagen than PRP which is unresponsive to arachidonate. Pre-incubation of PRP with epinephrine uniformly transforms PRP which does not aggregate on exposure to arachidonate alone into arachidonate-aggregating PRP. Dog PRP which aggregates with arachidonate release 14C-serotonin, while non-aggregating PRP does not. However, arachidonate stimulates malondialdehyde production in both aggregating and non-aggregating PRP. The results of this study indicate that dogs are heterogeneous in regard to their aggregation response to arachidonate. The mechanism of this heterogeneity is unknown: however, since prostaglandin metabolism is intact and the platelets of some dogs respond to arachidonate alone, it appears to be the result of variable sensitivity to endoperoxides and thromboxane A2.