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Objective To determine the e¡ect of two doses of fentanyl, administered transdermally, on the minimum alveolar concentration (MAC) of iso£urane in cats.Study design Prospective, randomized study.Animals Five healthy, spayed, female cats.Methods Each cat was studied thrice with at least 2 weeks between each study. In study 1, the baseline iso£urane MAC was determined in triplicate for each cat. In studies 2 and 3, iso£urane MAC was determined 24 hours after placement of either a 25-or 50-mg hour À1 fentanyl patch. In each MAC study, cats were instrumented to allow collection of arterial blood and measurement of arterial blood pressure. Twenty-four hours prior to studies 2 and 3, a catheter was placed and secured in the jugular vein and either a 25 or 50-mg hour À1 fentanyl patch was placed in random order on the left thorax. Blood samples for plasma fentanyl determination were collected prior to patch placement and at regular intervals up to 144 hours. After determination of MAC in studies 2 and 3, naloxone was administered as a bolus dose (0.1 mg kg À1) followed by an infusion (1 mg kg) and MAC redetermined.Results The baseline iso£urane MAC was 1.51 AE 0.21% (mean AE SD). Fentanyl (25 and 50 mg hour À1 ) administered transdermally signi¢cantly reduced MAC to 1.25 AE 0.26 and 1.22 AE 0.16%, respectively. These MAC reductions were not signi¢cantly di¡er-ent from each other. Iso£urane MAC determined during administration of fentanyl 25 mg hour À1 and naloxone (1.44 AE 0.16%) and fentanyl 50 mg hour À1and naloxone (1.51 AE0.19%) was not signi¢cantly di¡erent from baseline MAC (1.51 AE0.21%).Conclusions and clinical relevance Fentanyl patches are placed to provide long-lasting analgesia. In order to be e¡ective postoperatively, fentanyl patches must be placed prior to surgery. Plasma fentanyl concentrations achieved intraoperatively decrease the need for potent inhalant anesthetics in cats.