Oxygen consumption (QO) was Measured polarographically in mouse diaphragms stimulated electrically in the presence of (+)-tubocurarine. The preparations showed the basal QO. Electrical stimulation facilitated the rate of QO. The effect was evoked repetitively with a small decline. KCN (1 mM) abolished both the basal and the facilitated rates of QO. The facilitated rate of QO depended on the frequency of the stimulation (0.1-3.2 Hz). Under continuous stimulation at 0.1 Hz, veratridine (300 nM) increased the rate of QO with an increment of twitch tension. These effects of veratridine were reduced by ouabain (10 μM). Residual but considerable rates of the facilitated QO were still measured in preparations in which twitch contractions were blocked by dantrolene Na (100 μM) or ethylene glycol (1095 mM). Ouabain (10 uM) and tetrodotoxin (1 uM) reduced the residual facilitation of QO. Veratridine alone did not alter tissue content of Na. Ouabain significantly increased the tissue level of Na. This effect of ouabain was further potentiated by coexistence of veratridine. These results suggest that facilitation of Na-permeability across the cell membrane potentiates the rate of QO and that the inhibition of Na-K-ATPase and/or accumulation of Na in the cell reduce the facilitation of QO induced by electrical stimulation.