Objective: To evaluate the effects of a constant rate infusion (CRI) of romifidine on the requirement of isoflurane, cardiovascular performance and recovery in anaesthetized horses undergoing arthroscopic surgery. Study design: Randomized blinded prospective clinical trial. Animals: Thirty horses scheduled for routine arthroscopy. Methods: After premedication (acepromazine 0.02 mg kg-1, romifidine 80 mu g kg-1, methadone 0.1 mg kg-1) and induction (midazolam 0.06 mg kg-1 ketamine 2.2 mg kg-1), anaesthesia was maintained with isoflurane in oxygen. Horses were assigned randomly to receive a CRI of saline (group S) or 40 mu g kg-1 hour-1 romifidine (group R). The influences of time and treatment on anaesthetic and cardiovascular parameters were evaluated using an analysis of variance. Body weight (t-test), duration of anaesthesia (t-test) and recovery score (Wilcoxon Rank Sum Test) were compared between groups. Significance was set at p