Abstract Objective—To quantitate the dose and time-related effects of morphine sulfate on the anesthetic sparing effect of xylazine hydrochloride in halothane-anesthetized horses and determine the associated plasma xylazine and morphine concentration-time profiles. Animals—6 healthy adult horses. Procedure—Horses were anesthetized 3 times to determine the minimum alveolar concentration (MAC) of halothane in O2 and characterize the anesthetic sparing effect (ie, decrease in MAC of halothane) by xylazine (0.5 mg/kg, IV) administration followed immediately by IV administration of saline (0.9% NaCl) solution, low-dose morphine (0.1 mg/kg), or high-dose morphine (0.2 mg/kg). Selected parameters of cardiopulmonary function were also determined over time to verify consistency of conditions. Results—Mean (± SEM) MAC of halothane was 1.05 ± 0.02% and was decreased by 20.1 ± 6.6% at 49 ± 2 minutes following xylazine administration. The amount of MAC reduction in response to xylazine was time dependent. Addition of morphine to xylazine administration did not contribute further to the xylazine-induced decrease in MAC (reductions of 21.9 ± 1.2 and 20.7 ± 1.5% at 43 ± 4 and 40 ± 4 minutes following xylazine-morphine treatments for low-and high-dose morphine, respectively). Overall, cardiovascular and respiratory values varied little among treatments. Kinetic parameters describing plasma concentration-time curves for xylazine were not altered by the concurrent administration of morphine. Conclusions and Clinical Relevance—Administration of xylazine decreases the anesthetic requirement for halothane in horses. Concurrent morphine administration to anesthetized horses does not alter the anesthetic sparing effect of xylazine or its plasma concentration-time profile. (Am J Vet Res 2004; 65:519–526)