AbstractBackgroundAcidemia in sick or injured horses is often due to lactic acid accumulation. Alterations in platelet function and hemostasis are among numerous deleterious effects caused by decreased physiologic pH.ObjectivesWe aimed to evaluate the effect of hyperlactatemia and resultant acidemia on platelet aggregation in equine whole blood using impedance aggregometry.MethodsPlatelet aggregation was measured using the Multiplate analyzer in whole blood from 34 healthy horses at baseline and after in vitro addition of lactic acid to adjust the pH. Platelet aggregation of each sample was quantified by the area under the curve measurement reported by the Multiplate system. The association between platelet aggregation and pH was analyzed using a linear mixed‐effects model. The association of baseline platelet aggregation with hematocrits (Hcts), white blood cell (WBC) counts, and platelet counts was evaluated using Pearson's correlations.ResultsThere was a significant association between acidemia and decreased platelet aggregation. No significant correlations were detected between platelet aggregation and Hct, WBC count, or platelet count. Platelet aggregation measured in healthy horses using the Multiplate analyzer showed substantial variation between animals.ConclusionsAcidemia caused by the addition of lactic acid to equine whole blood was associated with a mild though statistically significant decrease in platelet aggregation. In conjunction with other factors, this change may contribute to morbidity‐related disorders of hemostasis, although its precise clinical relevance is uncertain.