Energy metabolism and body temperature were examined in Leontopithecus rosalia, the golden lion tamarin. Total standard metabolic rate (SMR), defined as the metabolic rate of resting, fasted animals within thermoneutrality and during the inactive (nighttime) phase, averaged 381.5 +/- 65.2 ml O2.h-1 (mass-specific metabolic rate 0.520 +/- 0.089 ml O2.g-1.h-1). This value ranges from 73 to 89% of the expected SMR for animals of this body size depending on the predictive equation used. Active-phase resting metabolic rate within thermoneutrality was significantly greater than SMR, averaging 509.0 +/- 44.6 ml O2.h-1 (0.709 +/- 0.062 ml O2.g-1.h-1). Thermal conductance during the inactive phase was 20.3 +/- 2.7 ml O2.h-1.degrees C-1 (0.029 +/- 0.003 ml O2.g-1.h-1.degrees C-1) or 70% of that during the active phase (28.5 +/- 3.2 ml O2.h-1.degrees C-1, 0.042 +/- 0.004 ml O2.g-1.h-1.degrees C-1). These values are about 85% of the mammalian predicted value. Body temperature fluctuated substantially between day (39.6 degrees C) and night (37.4 degrees C). However, none of these differences between circadian phases are unusual for primates (or, indeed, mammals) of similar body size. Although the metabolic rate of Leontopithecus is lower than the predicted one, it is higher than those rates reported for small nocturnal primates with similar food habits. Leontopithecus' modest rates of energy turnover may reflect a combination of phylogenetic constraints, feeding ecology and/or an energy-saving tactic that comprises part of a strategy to maximize reproductive effort. Although callitrichids are often cited as having rapid growth and high reproductive effort, the moderately low SMR of Leontopithecus is consistent with its intrinsic rate of natural increase, which is 90-94% of that expected for a mammal of its body size.