this study aimed to compare the sedative effects of morphine, meperidine and fentanyl, in combination with acepromazine (acp) and their effects on physiologic values in dogs. six healthy beagle dogs were randomly assigned to four treatments with 7-day washout intervals. in three treatments, acp (0.05mg kg-1) was administered and 20 minutes later, the dogs received administration of 0.5mg kg-1 of morphine (acpmor), 5mg kg-1 of meperidine (acpmep) or 5μg kg-1 of fentanyl (acpfen). in treatment acphdmor, 0.1mg kg-1 of acp was administered in combination with 0.5mg kg-1 of morphine. all drugs were administered intravenously. sedation scores were evaluated by a numeric descriptive scale (nds: 0-3) and a simple numeric scale (sns: 0-10). all variables were evaluated for 120 minutes. the administration of acp caused mild to moderate sedation. sedation was improved in all treatments after opioid administration, but significant differences were detected only in acpmor and acphdmor. more dogs presented intense sedation (nds=3.0) after administration of morphine (3/6 and 4/6 dogs in acpmor and acphdmor versus 1/6 in other treatments). duration of sedation was longer in acpmor and acphdmor. mild to moderate decreases in blood pressure, respiratory rate and temperature were observed in all treatments but decreased hr was observed only in acpmor and acphdmor. no significant differences were observed in the aforementioned variables when twice the dose of acp was used (treatment acphdmor). under the conditions of this study, administration of morphine, in combination with acp, results in greater and longer sedation than meperidine and fentanyl. increasing the dose of acp, in combination with morphine, does not improve the degree of sedation. all combinations used were considered to be safe for healthy dogs.