Endorphins are hormones released into your bloodstream when you engage in moderate to intense exercise for a fairly long period of time (can vary person to person). Keeping the pace where it is difficult to breathe for a certain period of time can bring on this "runner's high". I rarely push myself to the point where I feel the rush (high) of endorphins, but I did today! I did the eliptical machine at the gym at an intense level for 40 minutes and some strength machines + sit-ups and push-ups and I felt it! Exhilerating plus you feel so peaceful for a while. I am at the point where I can exercise at this intense level now-- I love it!