A digital ambulatory recording system of electrodermal activity (D-EDA) that uses a multimedia card (MMC) as a compact data storage device was developed. Low power consumption allows 24 h simultaneous recording of skin conductance and/or skin potential. A reduction in weight and size of the system provided a more natural environment for ambulatory recording. Direct A/D conversion and storage to MMC remarkably simplified data processing. Improvement in time and amplitude resolution allowed observation during sleep of even fine fluctuations in skin potential and skin conductance, which had been buried in the rolling noise generated by our former analog cassette data recorder.