The developmental changes of the cell membrane electrical properties were studied with micro-electrodes in a rat skeletal muscle cell line. 2. The resting potentials in myoblasts were minus 71 plus or minus 3 mV (mean plus or minus S.D.) and those in myotubes which were formed by fusion of myoblasts were minus 69 plus or minus 3 mV. There was no developmental change in the resting potential during the period examined. 3. The ionic mechanism for the resting potential was the same in myoblasts and myotubes. The resting membrane was almost exclusively permeable to K ions, while permeability to Na ions was not detectable. There was a small permeability to Cl ions. 4. The specific membrane resistance and capacitance were 8 k omega. cm-2 and 1 muF/cm-2 for myoblasts and 12 k omege. cm-2 and 5 muF/cm-2 for myotubes, respectively. 5. Action potentials in myoblasts were evoked by anode break stimulation. They were small and did not overshoot zero membrane potential. The action potentials in myotubes were larger, and had an average overshoot of 32 plus or minus 7 mV and a maximum rate of rise of 93 plus or minus 28 V/sec. 6. The current-voltage relation was examined. Delayed rectification was found in myotubes but not in myoblasts.
