Two different ATP extraction methods, one using perchloirc acid and the other boiling Tris bufer, for ATP determinations in myocardial samples werer investigated and compared. A bioluminescent assay was used to quantify the extracted ATP. In spite of some drawbacks, eprchloric acid extraction is zix times more efficient thatn boiling Tris buffer extraction for releasing ATP from myocardial tissue. The perchloric acid extraction was used to determine ATP depletion in the endocardium of isolated ischemic rat hearts.