He started by devising a simple experiment with a small group of subjects who ran or bicycled for 40 minutes at 76 percent of their max heart rate, and then had blood samples drawn immediately after exercising. Next the blood samples were flown to a special lab in Irvine, California. The results showed that both the runners and bicyclists had significantly more anandamide in their blood after exercising, with the greatest increase among the runners.
http://www.runnersworld.com/runners-stories/runners-high