To characterize the isoflurane-sparing effects of a high and a low dose of fentanyl in dogs, and its effects on mean arterial pressure (MAP) and heart rate (HR).Prospective, randomized crossover trial.Eight healthy male Beagle dogs weighing 12.1 ± 1.6 kg [mean ± standard deviation (SD)] and approximate age 1 year.Dogs were anesthetized using isoflurane and minimum alveolar concentration (MAC) was determined in duplicate by the bracketing method using an electrical stimulus on the tarsus. Animals were administered fentanyl: low dose (33 μg kg-1 loading dose, 0.2 μg kg-1 minute-1) or high dose (102 μg kg-1 loading dose, 0.8 μg kg-1 minute-1) and MAC was re-determined (MACISO-F). Blood was collected for analysis of plasma fentanyl concentrations before administration and after MACISO-F determination. All values are presented as mean ± SD.Isoflurane MAC (MACISO) was 1.30 ± 0.23% in the low dose treatment, which significantly decreased to 0.75 ± 0.22% (average MAC reduction 42.3 ± 9.4%). MACISO was 1.30 ± 0.18% in the high dose treatment, which significantly decreased to 0.30 ± 0.11% (average MAC reduction 76.9 ± 7.4%). Mean fentanyl plasma concentrations were 6.2 and 29.5 ng mL-1 for low and high dose treatments, respectively. MAP increased significantly only in the high dose treatment (from 81 ± 8 to 92 ± 9 mmHg). HR decreased significantly in both treatments from 108 ± 25 to 61 ± 14 beats minute-1 with the low dose and from 95 ± 14 to 42 ± 4 beats minute-1 with the high dose.Fentanyl administration resulted in a dose-dependent isoflurane MAC-sparing effect with bradycardia at both doses and an increase in MAP only at high dose. Further evaluation is needed to determine the effects of fentanyl on the overall cardiovascular function.