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1. The thermal dependence of the resting potential of isolated barnacle muscle fibres was larger (1-2 mV/° C) than predicted by Nernst's equation (about 0·2 mV/° C). A comparative study was made of the influence on thermal dependence of parameters related to (a) passive permeability and to (b) Na extrusion.

2. High [K]_o decreased the thermal dependence reversibly. [K_i], [Na]_i and [Cl]_i were determined by chemical analysis, and Goldman's equation was fitted to data relating V to [K]_o at different temperatures, in the presence and absence of ouabain 5 x 10^-5 M. In both cases the behaviour of V when T was lowered from 20 to 4° C was accounted for by increases in the calculated P_Na/PK and P_Cl/PK (from 0·006 to 0·043 and from 0·17 to 0·34 on the average, respectively.)

3. Other parameters related to passive permeability (and which caused reversible depolarization): decreased [Cl]_o (methanesulphonate or gluconate substituted), and decreased pH_o (below 5·0), also decreased the thermal dependence reversibly.

4. Inhibitors (ouabain 5 x 10^-5 M, cyanide 2-10 x 10^-3 M, 2,4-dinitrophenol 2 x 10^-4 M) externally applied did not affect either resting potential or its thermal dependence for several hours.

5. Increasing [Na]_i three- to fourfold by intracellular injection decreased both resting potential and its thermal dependence.

6. Although a small effect by a Na electrogenic pump cannot be excluded, the largest part of the thermal effect on the resting potential is concluded to depend on temperature-induced variations in relative ionic permeabilities to cations and anions. A model is proposed which can account for the data assuming that (a) each permeant ion associates to a separate site in the membrane, and (b) the ion-site equilibrium is temperature-dependent.