The influence of atropine on anesthesia induced by xylazine-pentobarbital administration was studied in 5 dogs. The combination of xylazine (2.2 mg/kg of body weight, IM) and pentobarbital (14.0 mg/kg, IV) caused anesthesia with the duration of absence of the pedal reflex, duration of anesthesia, and the time from return of consciousness to ambulation to be 107.4, 123.4, and 59.2 minutes, respectively. Bradycardia and short-term respiratory depression were observed. An IM injection of atropine sulfate (0.045 mg/kg) did not significantly change the durations of absence of the pedal reflex and of anesthesia, the time from return of consciousness to ambulation, or the pattern of respiration in the anesthetized dogs. The PaO2 increased gradually in both groups; however, atropine caused a marked tachycardia and increased the PaCO2. Fifteen minutes after pentobarbital injection, administration of atropine sulfate caused a slight but significant (P less than 0.01) decrease in arterial pH. Although atropine sulfate antagonized xylazine bradycardia, the data indicated that it may have caused increased respiratory depression in dogs anesthetized with xylazine and pentobarbital.