A new behavioral test was developed to investigate the neural mechanisms of voluntary, behavioral thermoregulatory responses. The apparatus used in this test consisted of a thermostatic chamber that maintained the ambient temperature at a chosen level and two side-by-side floor plates that were placed in the thermostatic chamber and could be set to different temperatures. As the three temperatures, ambient temperature and two plate temperatures, can be controlled independently, we term this behavioral test the three-temperature (3 T) test. When the ambient temperature was 28 °C with floor plate temperatures of 25 °C and 35 °C, mice showed preference to the warm plate over the cool one. By contrast, when the ambient temperature was 40 °C, the mice showed preference to the cool plate, that is, a cool-seeking behavior. Detailed analyses of the time courses of the plate preference and core body temperature revealed that this cool-seeking behavior contributed to the regulation of body temperature. By using the 3 T test in combination with the latest in vivo imaging techniques for real-time measurement of neuronal activities and neurotransmitter releases in the brain of freely-moving animals, the neural mechanisms of voluntary, behavioral thermoregulatory responses could be elucidated in the near future.