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.