The effectiveness of isoflurane as an inhalation anesthetic for muskrats (Ondatra zibethicus) was evaluated. Thirty muskrats were anesthetized in an enclosed chamber using 10 ml of isoflurane in Carlton County, Minnesota (USA), from 27 September to 24 October 1994. Mean (+/- SE) induction time for adults, juveniles, and kits was 20.6 +/- 2.9, 21.5 +/- 2.4, and 16.9 +/- 5.3 min, respectively (P = 0.77). Respective mean arousal times for adults, juveniles, and kits were 5.1 +/- 0.5, 5.7 +/- 0.7, and 5.7 +/- 0.6 min (P = 0.78). Heart rate, respiration rate, and body temperature were similar among age classes (P = 0.08 to 0.58). Mortality (3.3%) was comparable to that of other inhalation anesthetics. No short-term adverse effects were observed in recaptured individuals. Isoflurane is a safe and effective inhalation anesthetic for muskrats, although prolonged induction may limit its use in field studies.