In studies of energy balance it is of utmost importance to measure with a certain degree of accuracy the energy expenditure. So far the energy cost has been measured by taking accurate diaries of various activities performed during a day. This study tries to find the possibility to measure the energy cost through heart rate, since it is correlated with oxygen consumption. In 17 obese and non obese healthy subjects the relation between oxygen consumption and heart rate has been studied minute by minute during various physical activities. Data show the existence of a very close relationship between the two variables demonstrating that continuous heart rate monitoring (dynamic ECG) may be considered as a valid tool for energy expenditure measurements. Since each subject shows his own linear equation, it is necessary to assess it before the measurement.