Abstract Objective—To compare hemodynamic effects in dogs anesthetized with remifentanilisoflurane and with isoflurane alone. Animals—6 adult dogs. Procedures—Mechanically ventilated, isoflurane-anesthetized dogs received increasing constant rate infusions (CRIs) of remifentanil (0.15, 0.30, 0.60, and 0.90 μg/kg/min) or physiologic saline (0.9% NaCl) solution (control treatment), with a 1-week washout interval between treatments. Each CRI of remifentanil or saline solution was maintained for 60 minutes with equipotent end-tidal isoflurane concentrations that corresponded to 1.3 times the minimum alveolar concentration. Hemodynamic measurements and plasma vasopressin concentrations were determined before and at the end of each CRI and 60 minutes after the end of the infusion regimen. Results—Compared with the control treatment, remifentanil CRIs significantly decreased heart rate (HR) and cardiac index (CI) and significantly increased systemic vascular resistance index (SVRI) and plasma vasopressin concentration. Greatest differences in mean values between treatments were recorded for remifentanil at 0.60 μg/kg/min (HR and Cl were 55% and 47% lower, respectively, and SVRI was 91% higher than for the control treatment). Mean arterial pressure increased significantly during the highest remifentanil CRI (9% higher than for the control treatment). The increase in vascular resistance was positively correlated with increases in vasopressin concentrations (coefficient of determination, 0.65) during anesthesia with remifentanil-isoflurane. Conclusions and Clinical Relevance—Anesthesia maintained with remifentanil-isoflurane may decrease tissue perfusion as a result of a decrease in Cl. However, hypotension may not develop because of systemic vasoconstriction. An increase in plasma vasopressin concentration was associated with the vasoconstriction observed in dogs anesthetized with remifentanil-isoflurane. (Am J Vet Res 2010;71:1133-1141)