OBJECTIVE : To evaluate the effect of two doses of fentanyl upon chest wall rigidity of dogs anesthetized at equipotent doses of isoflurane [1.3 minimum alveolar concentration (MACISO) of each dose of fentanyl]. METHODS : Prospective crossover randomized study. METHODS : A group of eight male Beagle dogs, approximately 1 year old and weighing 12.1 ± 1.6 kg (mean ± standard deviation). METHODS : The dogs were anesthetized with isoflurane and instrumented for the measurement of esophageal pressure (PESO), flow (V˙) and volume (V). Chest wall elastance (ECW) was estimated by multiple linear regression of the model. PESO(t) = V˙(t) × RCW + V(t) × ECW + EEPESO where t is time, RCW is chest wall resistance and EEPESO is end-expiratory PESO. Chest wall compliance (CCW) was calculated as 1/ECW and normalized to the body weight of each dog (mL cmH2O-1 kg-1). Anesthesia was maintained at 1.3 MACISO for at least 15 minutes and CCW recorded (CCW-ISO). The dogs were randomly assigned to the lower fentanyl dose [loading dose (33 μg kg-1) and infusion (0.2 μg kg-1 minute-1)] or the higher fentanyl dose [loading dose (102 μg kg-1) and infusion (0.8 μg kg-1 minute-1)]. After 60 minutes of fentanyl infusion, CCW was recorded for each dose (CCW-FENT). During fentanyl infusion, the dogs were maintained at equipotent doses of isoflurane (1.3 MACISO for each fentanyl dose). A two-way analysis of variance followed by a Bonferroni test was used to compare CCW-ISO and CCW-FENT in both treatments and CCW-FENT between treatments. A p value <0.05 was considered significant. RESULTS : Neither of the fentanyl doses decreased CCW and there was no difference in CCW-FENT between doses. CONCLUSIONS : Fentanyl at the studied doses did not result in chest wall rigidity in dogs anesthetized with equipotent doses of isoflurane (1.3 MACISO).