Summary Our experiments indicate that the nucleotides of different tissues vary greatly in lability. When dog muscle was removed rapidly and minced in ice cold trichloroacetic acid, its nucleotide pattern resembled that seen in fresh blood, i.e., ATP greatly exceeded ADP or AMP. Delay in mincing the tissue caused some decrease in ATP with increases in ADP and AMP. On the other hand, dog or rabbit intestinal mucosa was so labile that usual procedure of removing tissue and transferring it immediately to cold trichloroacetic acid was much too slow. Injecting cold trichloroacetic acid into the live tissue was necessary to give a nucleotide pattern which resembled patterns seen in other tissues. Incubation under circumstances tried, were unsuccessful in restoring tissue nucleotide patterns. Extrapolation of foregoing results suggests that studies involving either distribution of nucleotides in tissues or determination of tissue nucleotides after in vitro incubation must indicate degree of lability of these tissue nucleotides.