Four groups of isolated rat livers underwent perfusion at 37 °C for 1 h, at 37, 40, 43 or 45 °C, respectively, during the 2nd hour and then at 37 °C again for the 3rd hour. Vascular resistance slightly decreased during hyperthermia and then significantly increased after restarting normothermic perfusion. At 40 ° C bile production, oxygen consumption, glucose and lactate release did not significantly differ from those found in the 37 °C group. At 43 and 45 °C all these parameters were significantly impaired when compared to the 37 and 40 ° C groups and did not recover after normothermic perfusion was restored. GOT and GPT release increased in proportion to the temperature. Microscopic examination revealed normal histologic features in 37 and 40 °C specimens while alterations such as vacuolization and focal necrosis were found in the 43 and 45 °C groups. These data indicate that the highest temperature that is well tolerated by isolated rat livers for 1 h is located between 40