The effectiveness of detomidine with or without atropine sulfate premedication in producing sedation and analgesia for arthrocentesis was studied in 12 horses. The effects were evaluated by monitoring heart and respiratory rates, borborygmi, distance from the lower lip to the floor, systolic blood pressure, and response to needle insertion. Either atropine or saline (as a placebo) was administered immediately prior to detomidine. All drugs were administered intravenously. Measurements were taken prior to drug injection and at 1, 5, 10, 15, 20, 25, 30, 40, 50, 60, 120, 180 and 240 minutes postinjection. Detomidine with atropine resulted in significantly higher heart rates than detomidine without atropine for the three hours of observation. Borborygmi were significantly decreased for four hours following detomidine with atropine and for three hours following detomidine without atropine, when compared to preinjection levels. Systolic blood pressure was significantly increased for 15 minutes following detomidine and atropine compared to the preinjection level. The head was markedly lowered for 60 minutes with either treatment. Atropine prevented the bradyarrhythmia and bradycardia induced by detomidine, but it induced a tachycardia. A satisfactory response for needle insertion and adequate synovial fluid aspiration was achieved in 95% of the trials with detomidine, with or without atropine sulfate premedication. The results suggest that, although atropine prevents bradyarrhythmia and bradycardia following detomidine, administering detomidine without atropine is satisfactory for arthrocentesis in untrained horses.