RIASSUNTO L’attività analgesica del fentanyl è associata ad effetti collaterali che devono essere tenuti in considerazione, soprattutto quando associato ad altri farmaci, come quelli anestesiologici, che provocano implicazioni nella dinamica cardio-circolatoria e respiratoria. Il presente lavoro, pertanto, valuta le variazioni dei parametri emodinamici e respiratori conseguenti alla somministrazione di fentanyl associato ad altri anestetici impiegati per interventi di ortopedia. Dai risultati ottenuti abbiamo verificato che la somministrazione di fentanyl non comporta modificazioni rilevanti dei parametri vitali, pur riducendo la percentuale di isoflurano erogato, e garantisce un’analgesia adeguata. SUMMARY Pre-emptive administration of analgesics may prevent dorsal horn hyperexcitability and wind-up. Fentanyl is a synthetic opioid agonist from 80 to 100 times more potent than morphine approximately, but having shorter duration of action. The side effects of fentanyl analgesic action must be taken in account above all when the fentanyl is associated to other drugs with similar effects. This works presents the results obtained about the changes in the emodynamic and respiratory parameters due to the administering of both fentanyl and another anaesthetics used during orthopedic operations. In this study we observed 6 dogs undergoing 90 minutes of orthopaedic surgery. The fentanyl administering does not involve considerable changes in the vital parameters even thought it lowers the administered forane percentage and ensures suitable analgesia. Considering efficacy, the progressive forane percentage reduction, shows a good analgesia achievement. The chosen protocol seems to be adequate according to an analgesic/anaesthetic and safety point of view, even though the excessive vocalizations caused more by the side effect of fentanyl than by pain