Oxygen consumption (Vo2) and body temperature (Tb) were measured in Manacus vitellinus (mean mass, 15.5 g) and Pipra mentalis (mean mass, 12.3 g) on Barro Colorado Island, Panama. The two species had the same mean euthermic nocturnal Tb (37.9 C). During activity Tb sometimes reached 43 C. In both species, nocturnal basal metabolic rate (BMR) (41.69 cm3 O2/h in M. vitellinus and 34.9 cm3 02/h in P. mentalis) was significantly less than predicted on the basis of mass. At night fasted birds frequently, and unfasted birds occasionally, became hypothermic, with Tb ranging between 27 and 36 C. Their Tb always remained several degrees above ambient temperature (Ta). The lowest Tb recorded was 26.8 C at a Ta of 14.6 C. Thermal conductance was the same in euthermic and hypothermic birds. Manakins save substantial amounts of energy by their nocturnal hypothermia. During a 12-h night a 14-g M. vitellinus maintaining a Tb of 27 C in a Ta of 22 C for 10 h would expend 6.4 kJ less than if it maintained its Tb at the mean euthermic nocturnal level. This represents a savings of 58%. The selective pressures that have favored nocturnal hypothermia in manakins have probably operated on other small tropical frugivorous birds. We predict that the main components of the metabolic pattern of manakins will also be found in other small tropical passerines with similar food habits.