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OBJECTIVE To determine the pharmacokinetics and effects on thermal thresholds (TT) of two fentanyl constant rate infusions in awake cats. STUDY DESIGN A blinded, randomized crossover study. ANIMALS A group of six healthy female cats, aged 3 ± 1 years, weighing 4.1 ± 0.7 kg. METHODS Skin temperature (TSKIN) and TT were evaluated using a wireless TT device. TSKIN, TT, sedation score (SS) and blood samples were collected before an intravenous loading dose (LD; over 5 seconds) and at specific time points during (360 minutes) and after infusion. Each cat was administered two treatments: fentanyl (LD 3 μg kg-1, infusion 3 μg kg-1 hour-1; treatment F3) or fentanyl (LD 5 μg kg-1, infusion 5 μg kg-1 hour-1; treatment F5). SS between treatments was analyzed using a Kruskal-Wallis test. Statistical analysis of TT and TSKIN was performed using analysis of variance with appropriate post hoc test (p < 0.05). RESULTS TSKIN did not vary over time for each treatment. SS did not differ between treatments. TTs were significantly higher than baseline at 15 minutes after LD for F3 and F5. TT was significantly increased at 30, 90, 120, 180 and 300 minutes in treatment F5 but not in F3. Plasma fentanyl concentrations decreased rapidly in both treatments over the first 30 minutes after infusion. The terminal half-life was 3.31 (2.93-4.41) hours for F3 and 3.67 (3.39-4.32) hours for F5 (median, range). Systemic clearance for treatments F3 and F5 was 1.95 (1.46-2.44) and 2.25 (1.98-2.47) L hour-1 kg-1 (median, range), respectively. Plasma concentrations <1.84 ng mL-1 were not associated with a significant increase in TT. CONCLUSIONS and clinical relevance A fentanyl infusion rate of 5 μg kg-1 hour-1 increased TT during the infusion period. Effects on TT were lost rapidly with cessation of the infusion.