Fatiguing stimulation of isolated rat skeletal muscle leads to depolarization of the cellular membrane and a considerable loss of force (1) suggesting excitability of the membrane as an important mediator of the fatigue. Na⁺/K⁺– pump activity increases dramatically in response to β₂-agonist addition, improving restoration of the Na⁺, K⁺ homeostasis and the contractile performance of a working muscle. Reduced availability of oxygen to the muscles has been shown to increase the rate of fatigue in isolated muscle (2) and isolated diaphragm muscle strips (3). The present study examines the effect of β₂-agonists on force in anoxic rat EDL muscle and the role of the Na⁺/K⁺– pumps in mediating this effect. EDL muscles were prepared from 4 wk old Wistar rats and mounted on holders for isometric contractions. Muscles were stimulated intermittently at 40 Hz for 15 min (10 sec on, 30 sec off, 10 V, 0.2 ms pulses) during anoxic conditions (95% N₂, 5% CO₂) and force recovery was followed during reoxygenation (95% O₂, 5% CO₂) for up to 240 min. β₂-agonists (salbutamol (10^-5 M) or salmeterol (10^-6 M)) were added either 15 min prior to the anoxic stimulation protocol or during recovery. Intracellular Na⁺ content was measured at the end of the reoxygenation period. The statistical significance of any difference between the control group and the treated group was ascertained using two-way Anova and the post hoc Bonferroni test. Stimulation during anoxia leads to rapid force decline and only partly recovery during the following reoxygenation period. Addition of salbutamol or salmeterol prior to the anoxic stimulation protocol improves force by 32 ± 10% (P<0.001, n=7) and 40 ± 7% (P<0.001, n=4), respectively, within the first 2-3 min of the fatiguing protocol, hereafter force declines rapidly in all groups. In the following reoxygenation period force in the control muscles reaches 27 ± 3% of initial force, whereas force in salbutamol treated muscles increases up to 42 ± 5% (P<0.05, n=7) of initial force. Addition of salbutamol to control muscles at 150 min in the reoxygenation period improves force by 102 ± 16% (P<0.001, n=12). Muscles treated with a β₂-agonist also show decreased intracellular Na⁺ content indicating that the Na⁺/K⁺-pumps are involved. Inhibition of the Na⁺/K⁺– pumps by ouabain or 2-deoxyglucose abolishes the positive effect of salbutamol on force. No detectable increase in the release of an intracellular enzyme (LDH) is observed during this anoxic protocol indicating that the observed force loss is not due to loss of cellular integrity. The main findings are that force during and following an anoxic period can be improved by addition of β₂-agonists (eg. salbutamol or salmeterol) most likely due to a stimulating effect on the Na⁺/K⁺– pumps.