An observational method for quantifying intensity of activity was validated against min-by-min heart rates during physical education periods. The mean heart rate values increased as the activity points increased. Moderate average correlations were obtained between the min-by-min activity points and heart rates. A time series regression analysis accounted for 72% of the variance in heart rate values with a model combining heart rate in the previous min and activity points. Further work on estimates of average metabolic units for activity categories will enable future investigators to obtain more precise estimates of energy expenditure from this observation system.