IN a previous study on the nucleotide metabolism of cardiac muscle1, the changes of adenosine triphosphate were investigated during systole and diastole. Using the Cohn and Carter method of analysis2 on a ‘Dowex-1’ column, the nucleotides were separated and determined quantitatively by ultra-violet absorption spectrophotometry. During the studies on the extracts of hearts frozen in systole by an acetone–dry ice mixture, a substance appeared as a new peak in the chromatogram, which seemed to be an adenine nucleotide with more nitrogens than could be accounted for. This was called nucleotide H.