A physiological strain index (PSI) based on rectal temperature (Tre) and heart rate was recently suggested to evaluate exercise/heat stress. This review summarizes the development of PSI which was evaluated from seven different databases during the last three years. PSI was developed from data obtained from man performing exercise in the heat and from a separate database of man wearing protective clothing and exercising in hot-dry and hot-wet environmental conditions. This index was also evaluated for heat strain associated with hypohydration and for exercise/heat stress and gender. In addition, PSI was adjusted for values in rats, and could be used to assess the individual roles played by heat acclimation and exercise training in rats. This simple to use index scales strain to a range of 0-10, can be used on-line or during data analysis, and includes the ability to depict rest and recovery periods. The PSI has the potential to be widely accepted and to serve universally.