The mean core temperatures (Tc) of two Amazonian manatees (Trichechus inunguis) were 35.6 ± 0.1 C (X ± SD) and 36.1 ± 0.2 C at ambient water temperatures (Tw = 27–29 C). Based on metabolic rate (MR), the lower critical water temperature (TLC) of the larger manatee was 22-23 C. Above TLC, Tc varied with Tw, but the animal maintained Tc below TLC. The increase in MR below TLC was primarily due to activity. Changes in Tc lagged 3–4 h behind the changes in Tw, while the changes in conductance (G) paralleled the changes in Tw. The results of this study indicate that Amazonian manatees have a limited capacity for thermogenesis, and that their primary mechanism for thermoregulation is changes in the peripheral circulation. The possible role of subcutaneous fat in reducing peripheral heat loss is also discussed.