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The aim of this study was to evaluate the effect of hydrocortisone on adenosine diphosphate (ADP)-induced platelet aggregation in horse. The study was carried out on 35 healthy horses of various breeds and gender. On all horses, citrated blood samples were collected by jugular venipuncture to assess platelet aggregation. On all samples, platelet-rich and platelet-poor plasma were prepared by centrifugation and divided into two different aliquots. On the first aliquot, platelet aggregation was determined after platelet activating with 1 and 0.5 μM ADP (group A). On the last one, the effect of 20-min preincubation with hydrocortisone in ADP-induced aggregation was determined at final ADP concentrations of 1 and 0.5 μM (group B). By means of an aggregometer, the platelet response was quantitated as maximum degree of platelet aggregation and initial velocity of platelet aggregation. All data were analyzed using unpaired Student's t test. A P ≤ 0.05 was considered statistically significant. The obtained results showed a statistically significant effect of hydrocortisone on both the maximum degree of platelet aggregation and the slope of platelet aggregation at the two final concentrations of ADP used. These results are compatible with the possibility that the vessel wall releases smooth muscle-relaxing prostaglandins when injured and that the inhibition of prostaglandin formation by hydrocortisone enhances hemostasis by allowing vasoconstriction to be maintained. Further investigations could be useful utilizing different concentrations of hydrocortisone in order to apply the knowledge of the effect of glucocorticoids on platelet aggregation. [ABSTRACT FROM AUTHOR]