Data collected from experiments involving (1) intermittent cycling and load carrying in dry, neutral to hot, ambient temperatures and (2) prolonged walking (3 to 3.5 mph) in hot, humid environments were used for correlation analysis between some measured physiological variables. Increasing the air temperature (Ta) resulted in (1) a shift of regression line of heart rate (HR) on oxygen uptake (Vo2) such that, for a given Vo2, HR was 10 beats/min higher for a rise of 10 °C in Ta, and (2) apparent slope for the linear regression of the HR during the last minute of work on the HR during the first minute of recovery becoming less steep with rise in Ta. For prolonged work loads requiring Vo2 of about 1 liter/min (representing about 30% of maximal Vo2) amd Vo2 of about 1.5 to 2.8 liters/min (representing 40 to 60% of maximal Vo2) the regression of rectal temperature on HR was 0.03 °C and 0.01 °C per beat per minute respectively. It was concluded that the rate of change rather than absolute values of these parameters might provide a better guide for evaluation of strain, particularly when intermittent type of work is involved.