Intact rat soleus muscles or hemidiaphragms were incubated at 37°C for 1 h in 1.0 ml of Krebs-HCO3 buffer containing 10 mM glucose, 5 mM (8-14C)-labelled ATP and (α-32P)-labelled ATP together with 5 mM MgCl2 under an atmosphere of 95% O2-5% CO2. Samples of the incubation medium and tissue extract were subjected to electrophoretic separation and the radioactivity present as adenine nucleotides was counted. Extensive degradation of the added nucleotides was observed in the presence of both tissues. The concentrations of 14C-labelled and 32P-labelled ATP found in the muscle and diaphragm indicated that ATP was present within the muscle and diaphragm cells. By maintaining higher concentrations of ATP in the medium, ATP uptake in both tissues exhibited a saturation-type kinetics. These results provide further evidence for intracellular uptake of ATP and also suggest that the transport of ATP into the cell could be a carrier-mediated process. © 1980.