1. Extracellularly applied adenosine 5'-triphosphate (ATP) is known to have an excitatory action on chick skeletal muscle. By making intracellular recordings from cultured chick myotubes bathed with blockers of several types of voltage-dependent channels, the direct action of ATP could be observed. 2. When muscle cells were studied near their resting potential, ATP usually produced a biphasic response. There was a rapid initial depolarization, followed by a slower repolarization. The repolarization could drive cells negative to their initial resting potential, indicating that it was not due simply to desensitization of the process that produced the depolarization. Thus there are at least two distinct responses to ATP. 3. At room temperature the early response to ATP activated within 20 ms, and the second response activated with a latency of approximately 1 s. In our standard blocking solution, the average reversal potential of the early response was -17 mV, while the late response had a reversal potential that was negative to -70 mV. In a few cells the second response appeared to be absent. 4. The amplitude and time course of the late response were substantially decreased by low temperature (12 degrees C) and increased by high temperature (37 degrees C). In contrast, temperature had much smaller effects on the early response. Both the time course and temperature dependence of the late response suggest that an intracellular second messenger system may be involved in its activation. 5. Ion-substitution experiments were performed to determine the type of conductance changes that evoke each response. These indicated that the early response was due to an increased membrane permeability to sodium, potassium and chloride, but not to large cations or anions, and that the late response was due to an increased permeability to potassium. 6. Measurement of the responses of muscle cells to acetylcholine supported the conclusion that both anions and cations are permeable during the early ATP response. Under conditions in which there was a large negative reversal potential for all cations, and a large positive reversal potential for all anions, the early ATP response reversed approximately 50 mV positive to the acetylcholine response. 8. The possibility that the early ATP response is due to a channel selective for size, but not charge, is discussed.