Abstract Introduction: Recently we attempted an ECG recording in intact freely-behaving mice by using a plate sensor that has multiple (15) gold-plated electrodes, on which mice can walk around freely, without the use of tether or telemetry system. Although the electrodes are a kind of dry electrode, we found that the ECG plate sensor detects ECG from mouse’s footpads during sleep or even during wake-sleep transition probably because of the sweating from their footpads. Methods: A mouse cage, in which the ECG plate sensor was mounted, was placed in a Faraday cage. Fifteen ECG signals of the ECG plate sensor were fed to a PC via a 15-channel amplifier and a 16-channel A/D converter with using Clampex7 (Axon Instruments) signal acquisition software. Mouse behavior was also recorded by a DVD recorder. Intact, wild-type C57BL/6J mice were simply put on the ECG plate sensor for the recording for about 2 hours. Results: Although the ECG plate sensor could not detect ECG most of the period during active state, ECG signal appeared relatively stably during sleep state from 10 out of 12 mice examined; two mice did not fall asleep within 2 hours. The ECG appearance during sleep continued for a maximum of ~15 minutes and disappeared when they awoke. One mouse allowed us to record ECG from the period before sleep onset that appeared to be quiet waking state based on the images stored in the DVD recorder. Conclusion: The physiological role of the sweating during sleep in mice is inexplicable because most of mammals including mice have eccrine sweat glands that are considered to be activated by sympathetic nerve activity in response to emotional stressors, to which mice would not exposed during sleep. Nonetheless, the finding of the sweating during sleep in mice that enables noninvasive ECG recording may provide us a new tool to investigate cardiac and autonomic nervous system activities during sleep and wake-sleep transition states. Support (If Any): This work was supported by a JSPS KAKENHI Grant, Number 24590268, Tokyo, Japan.