The objective of the study was to assess the validity of an activity monitor (AM) within a psychophysiological study. The AM was based on four body-fixed accelerometers and discriminated postures, transitions, and dynamic activities. Three subjects participated in each of two 4-hr sessions. During each session, consisting of two protocols, ambulatory accelerometer and heart rate measurements were made. The output of the AM was compared with simultaneously recorded video tapes. An overall agreement between AM and video of 88 and 96% was found. The number of transitions and dynamic periods, and the duration of activities were well determined. Posture-related heart rate changes were demonstrated. A three-sensor configuration hardly influenced the validity scores. The AM appeared to be a valid instrument to quantify aspects of physical activity, and offers new possibilities for ambulatory psychophysiological research.