From today's New York Times, an article entitled Yes, Running Can Make You High, which proves what many of us runners already knew: the runner's high actually exist and it's fueled by endorphins.
A pain researcher in Germany used pre- and post-run PET scans to study endorphin binding in the brains of ten long distance runners. They also did psychological tests to measure mood states before and after a two hour run. They found that endorphins bound to the prefrontal cortex and the limbic system, and that the amount of binding correlated with elevated mood. The research subjects were blind to the purpose of the study.
Just something to think about the next time you lace up the Nikes.