1. 1. Rat soleus muscle was incubated in the presence of [8-14C] ATP, [8-14C]ADP or [8-14C]adenosine. Samples of incubation medium and muscle extract were subjected to electrophoretic separation and radioactivity present in ATP, ADP, AMP, IMP, adenosine, inosine and hypoxanthine was counted. 2. 2. Extensive degradation of the added nucleotides was observed. Whereas adenylate kinase and adenosine deaminase activities appeared in the incubation medium, the other enzymes responsible for ATP degradation did not, suggesting that they were located on the exterior surface of the muscle cell. 3. 3. The concentrations of 14C-labelled ATP and ADP found in the muscle indicated that these compounds were present within the fibres. Evidence is presented which suggests that ATP, and to a lesser extent ADP, entered the muscle as such and were not synthesized within the muscle from 14C-labelled adenosine. © 1970.