1. Soleus muscles of anaesthetized rats were stimulated tetanically (4 s at 20 Hz every 5 s for 5 min), following which the resting and action potentials were measured in surface fibres. 2. At the end of the stimulation period, the mean resting potential was found to have increased from a control value of ‐79.5 +/‐ 4.8 mV (mean +/‐ S.D.) to ‐90.5 +/‐ 6.3 mV. The hyperpolarization started to decline after 9 min but was still present at 15 min. 3. Associated with the membrane hyperpolarization was an increase in the mean amplitude of the muscle fibre action potential, from 82.2 +/‐ 10.8 to 96.8 +/‐ 10.0 mV. 4. Both the hyperpolarization and the enlargement of the muscle fibre action potential were abolished by 1.25 X 10(‐4) M‐ouabain, cooling the bathing fluid to 19 degrees C or removing K+ from the bathing fluid. 5. The results are explained in terms of an increase in electrogenic sodium pump activity resulting from tetanic stimulation. When the bathing fluid contained 20 mM‐K+, the mean resting potential of stimulated fibres was approximately ‐30 mV greater than that calculated from the Goldman‐Hodgkin‐Katz equation. 6. The increase in sodium pumping not only acts to restore the concentrations of Na+ and K+ on either side of the muscle fibre membrane, but, through its electrogenic effect, enables fibres to remain excitable during continuous contractile activity.