Genetically obese Zucker (Z) rats have been reported to display a body core temperature (T_b) that is consistently below that of their lean littermates. We asked the question whether the lower T_b was a result of deficits in thermoregulation or a downward resetting of the set point for T_b. For a period of 45 consecutive hours, lean and obese Z rats were free to move within a thermal gradient with an ambient temperature (T_a) range of 15-35°C, while subjected to a 12:12-h light-dark cycle. T_b was measured using a miniature radio transmitter implanted within the peritoneal cavity. Oxygen consumption (O₂) was measured using an open flow technique. Movements and most frequently occupied position in the gradient (preferred T_a) were recorded using a series of infrared phototransmitters. Obese Z rats were compared with lean Z rats matched for either age (A) or body mass (M). Our results show that obese Z rats have a lower T_b [37.1 ± 0.1°C (SD) vs. 37.3 ± 0.1°C, P < 0.001] and a lower O₂ (25.3 ± 1.9 ml · kg¹ · h¹) than lean controls [33.1 ± 3.7 (A) and 33.9 ± 3.9 (M) ml · kg¹ · h¹, P < 0.001]. Also, the obese Z rats consistently chose to occupy a cooler T_a [20.9 ± 0.6°C vs. 22.7 ± 0.6°C (A) and 22.5 ± 0.7°C (M), P < 0.001] in the thermal gradient. This suggests a lower set point for T_b in the obese Z rat, as they refused the option to select a warmer T_a that might allow them to counteract any thermoregulatory deficiency that could lead to a low T_b. Although all rats followed a definite circadian rhythm for both T_b and O₂, there was no discernible circadian pattern for preferred T_a in either obese or lean rats. Obese Z rats tended to show a far less definite light-dark activity cycle compared with lean rats.