Once a week for 4 weeks, 5 dogs were given IM injections of xylazine (2.2 mg/kg of body weight) followed in 10 minutes by IV injections of pentobarbital (14 mg/kg). The resultant duration of anesthesia, absence of pedal reflex, and time from return of consciousness to ambulation were consistent from week to week. The mean times were 137.3, 111.8, and 56.9 minutes, respectively. A second experiment using 5 other dogs was performed to evaluate the antagonistic effect of yohimbine on the anesthesia induced by the xylazine-pentobarbital combination. When yohimbine (0.1 mg/kg, IV) was administered 10, 60, and 120 minutes after the xylazine-pentobarbital injection (given as in the 1st experiment), it abolished or markedly reduced the duration of anesthesia, absence of pedal reflex, and the time from return of consciousness to ambulation. After being given yohimbine, the dogs had a smooth recovery without postanesthetic excitement. In experiment 3, IM xylazine injections caused bradycardia without changing mean arterial blood pressure. Subsequent IV pentobarbital administration abolished xylazine-induced bradycardia for approximately 20 minutes and decreased arterial blood pressure slightly and gradually. Respiration was markedly depressed for the first 20 minutes of xylazine-pentobarbital anesthesia and gradually decreased during the rest of the 50-minute monitoring period. Yohimbine injection at postpentobarbital dosing minute 50 reversed the resumed xylazine-induced bradycardia and relieved other signs of respiratory depression associated with xylazine-pentobarbital anesthesia. The xylazine-pentobarbital combination was safe and effective for inducing and maintaining up to 2 hours of anesthesia in dogs.(ABSTRACT TRUNCATED AT 250 WORDS)