Body temperature is controlled by mechanisms that regulate heat production and heat loss. Apart from overt shivering, little is known about how muscle activity might contribute to regulation of body temperature around its set point. One contribution could be the long-lasting tonic motor unit activity which is caused by plateau potentials in motoneurones. Such potentials are generated by non-inactivating inward currents that can last many minutes. The potentials can be turned on or off by brief depolarizing or hyperpolarizing stimuli provided the motoneurones are also exposed to descending inputs from monoaminergic neurons in the brainstem. Presence of plateau potentials in motoneurones can be inferred from continuous EMG recordings via thin electrodes implanted in muscles such as the soleus (SOL). Here we present such recordings from unrestrained adult rats in a climatic chamber at four stable temperatures starting 3–5 days after electrode implantation. After 1–2 hrs of adaptation, continuous recordings lasting 4.5 hrs were made at 17, 20, 23, and 26 °C, increasing or decreasing in strict order from one day to the next. Recordings were performed at the same time of the day and under identical lighting conditions. Our results show that decreasing ambient temperatures cause graded increases in number and duration of tonic muscle activity episodes evidently arising from plateau potentials in SOL motoneurones. The response was reproducible within the same animal and between animals. We conclude that ambient-temperature dependent generation of plateau potentials in motoneurones to deep muscles with high resting blood perfusion rates such as the SOL evokes heat-producing tonic muscle activity that contributes to the fine-tuning of body temperature around its normal set point.