Endorphins
For example, in some studies a drug was given to people which blocked the effect of endorphins. These people still felt the runner's high; it means this feeling is not caused by the release of endorphins in the blood.  Another study was performed in 2004, which showed this feeling is related to a different body chemical named "anandamide".  Anandamide is not an endorphine but is an endocannabinoid, similar to one of the chemicals in marijuana and chocolate.