Anterograde amnesia (AA), forgetting of events that occur following a traumatic episode, has recently been demonstrated by using a mild decrease in temperature (hypothermia) as the amnestic agent. However, no data currently exist to indicate if an increase in body temperature (hyperthermia) might affect memory processing in a similar manner. Experiments 1 and 2 demonstrated that increasing the colonic body temperature of the rat to 3-4 °C or more above normal during avoidance training produced a significant retention loss when the test occurred 24 hr after training. Slight hyperthermia to 1-2 °C above normal did not impair retention. In Experiment 3, AA resulting from an elevation in temperature was reversed by reheating "amnestic" subjects just prior to the 24-hr test. By rapidly reversing hyperthermia immediately after the training trial with a cooling procedure, Experiment 4 demonstrated that hyperthermia-induced AA was not the result of retrograde influences of the heating treatment. Implications of these results are discussed in terms of possible retention deficits which could conceivably follow environmental heat stress or fever hyperthermia resulting from bacterial infection.