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1. The membrane characteristics of metabolically poisoned and mechanically exhausted frog skeletal muscle fibres were investigated with intracellular micro-electrodes. 2. When cyanide plus iodoacetate were applied as metabolic poisons twitch tension declined towards zero after 150-300 stimuli (0-3 Hz; temperature = 0 degrees C). At the beginning of stimulation the mean resting potenial fell from -75 to -69 mV; it rose subsequently to -83 mV. The membrane resistance decreased during this stimulation period along a sigmoid time course to 4-6% of the original value. 3. In completely exhausted fibres the following membrane constants were estimated (23 degrees C): length constant, 0-31 mm; input resistance, 31 komega; membrane resistance, 58 omega.cm2. These values were calculated under the assumption of a constant internal resistivity of 170 omega. cm. The Q10 values of these constants were similar to those in normal fibres. Afew experiments revealed that the membrane capacity remained roughly constant under these conditions. 4. The current-voltage relation of exhausted fibres was approximately linear in the range between -60 and -100 mV. At less negative potentials the conductance increased slightly while at more negative potentials it decreased. The latter, in particular, became more evident when the imput current was converted into membrane current density by applying Cole's theorem. 5TEA+ and Rb+ in the external solution increased the membrane resistance of exhausted fibres by more than one order of magnitude. The major part of the membrane conductance induced by exhaustion, however, could not be blocked by these ions or Zn2+. 6. Chloride-free test solutions were used to measure the relative contributions of potassium and chloride ions to the membrane conductance. The relation GK:GC1 changed from 2:3 in normal fibres to 5:1 in exhausted ones. In absolute terms GK rose from ca. 130 to 14,300 mumho/cm2 and GC1 from ca. 200 to 2900 mumho/cm2. The discrimination between K+ and Na+ by the resting membrane in exhausted fibres was probably equal to or even higher than that under normal conditions. 7. In normal fibres the input resistance decreased by up to 20% after the external application of 1-2 mM caffeine, which is known to release calcium ions from internal stores. The elevation in internal Ca2+ by direct injection caused a small and, as a rule, irreversible decrease in input resistance which was probably partly due to local damage to the surface membrane. 8. It is concluded that in metabolically exhausted muscle fibres the surface and tubular membranes are still intact and that the observed decrease in membrane resistance is mainly due to an increase in potassium conductance. In addition, the results indicate that the gating mechanism of the potassium channels (presumably those with the characteristics of the slow component) is affected when energy reserves diminish.

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