Scientists sometimes debate whether specific activities release measurable levels of endorphins. Much of the current data comes from animals which may not be relevant to humans. The studies that do involve humans often measure endorphin plasma levels, which do not necessarily correlate with levels in the central nervous system. Other studies use a blanket opioid antagonist (usuallynaloxone) to indirectly measure the release of endorphins by observing the changes that occur when any endorphin activity that might be present is blocked.