AbstractObjective—To quantitate dose- and time-related anesthetic-sparing effects of xylazine hydrochloride (XYL) during isoflurane-induced anesthesia in horses and to characterize selected physiologic responses of anesthetized horses to administration of XYL.Animals—6 healthy adult horses.Procedure—Horses were anesthetized 2 times to determine the minimum alveolar concentration (MAC) of isoflurane in O2and to characterize the anestheticsparing effect (MAC reduction) after IV administration of XYL (0.5 and 1 mg/kg of body weight, random order). Selected measures of cardiopulmonary function, blood glucose concentrations, and urinary output also were measured during the anesthetic studies.Results—Isoflurane MAC (mean ± SEM) was reduced by 24.8 ± 0.5 and 34.2 ± 1.9% at 42 ± 7 and 67 ± 10 minutes, respectively, after administration of XYL at 0.5 and 1 mg/kg. Amount of MAC reduction by XYL was dose- and time-dependent. Overall, cardiovascular and respiratory values varied little among treatments. Administration of XYL increased blood glucose concentration; the magnitude of change was dose- and time-dependent. Urine volume increased but not significantly.Conclusions and Clinical Relevance—Administration of XYL reduced the anesthetic requirement for isoflurane in horses. The magnitude of the decrease is dose- and time-dependent. Administration of XYL increases blood glucose concentration in anesthetized horses in a dose-related manner. (Am J Vet Res2000;61:1225–1231)