The resting ionic conductances of frog sartorius muscle fibers have been determined in a variety of conditions in order to measure the potassium conductance of the tubular and surface membranes (gKt and gKs) and the chloride conductance of the tubular and surface membranes (gClt and gCls). In both normal fibers and fibers without tubules, measurements of input resistance and diameter were made at normal pH and at low pH when the chloride conductance was very small. These measurements permitted the separation of the ionic conductances: gCls = 219 µmhos/cm2; gClt = 0 µmhos/cm2; gKs = 28 µmhos/cm2; gKt = 55 µmhos/cm2. Possible sources of systematic error are discussed and a statistical analysis of the effects of random error is presented. The implications of the nonuniformity of membrane properties are discussed along with possible anatomical explanations.

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