It has been known that platelet activating factor(PAF) plays some important roles in the development of allergic reactions. On the other hand, a close relationship between allergic reaction and platelet function was clarified in asthma and related diseases from the aspect of clinical laboratory findings. However, an accurate and direct role of platelet in these diseases was not clear. In this study, platelet was activated by PAF, and the release of a chemical mediator from the activated platelet was studied in order to study the direct action of platelet in allergic reactions. Furthermore, the effect of Azelastin hydrochloride, one of antiallergic drugs, on histamine release from the platelet of rabbits was studied. The results obtained from this experiment were as follows; Azelastin hydrochloride (with a final concentration of 2.5×10^-4M) inhibited PAF-induced aggregation of PRP of rabbits. In addition, Azelastin hydrochloride inhibited the release of histamine from washed platelet of rabbit activated by PAF in the same manner as inhibitory action of aggregation.