OBJECTIVE To evaluate the effects of premedication with acepromazine-morphine or acepromazine-methadone on the minimum alveolar concentration of isoflurane (ISOMAC) and the incidence of bradycardia and hypotension in dogs. METHODS Prospective randomized clinical study. METHODS Thirty-two female dogs undergoing elective ovariohysterectomy. METHODS Dogs were randomly assigned to one of three groups: no premedication (CONTROL group; n = 9); acepromazine (0.02 mg kg(-1)) and morphine (0.5 mg kg(-1)) (ACPMOR group; n = 11); and acepromazine (0.02 mg kg(-1)) and methadone (0.5 mg kg(-1)) (ACPMET group; n = 12). All drugs were administered intramuscularly. Twenty minutes later, anesthesia was induced with propofol administered intravenously to effect. Determinations of the ISOMAC were conducted by use of the up-and-down method using a quantal study design to determine the MAC for the population. Cardiovascular variables were registered immediately before noxious stimulation that was performed approximately 30 minutes after anesthetic induction. The occurrence of bradycardia (heart rates ≤ 70 beats minute(-1) in dogs ≤15 kg and ≤60 beats minute(-1) in dogs >15 kg) and hypotension (mean arterial pressure < 60 mmHg) were registered. RESULTS The ISOMAC in CONTROL was 1.20 ± 0.11%. Compared with CONTROL, the ISOMAC was reduced by 33.3% and 68.3% in ACPMOR and ACPMET, respectively (p < 0.001). The ISOMAC was lower in ACPMET than in ACPMOR (p < 0.001). Bradycardia was observed in 0%, 45% and 50% of dogs and hypotension was observed in 56%, 55% and 67% of dogs in CONTROL, ACPMOR and ACPMET, respectively. CONCLUSIONS The percentage reduction of the ISOMAC in ACPMET was approximately twice that in ACPMOR. Premedication with acepromazine-morphine or acepromazine-methadone increased the incidence of bradycardia. Hypotension was observed in most dogs during isoflurane anesthesia regardless of premedication.