Elevated plasma HCO₃^- can improve exercise endurance in humans. This effect has been related to attenuation of the work-induced reduction in muscle pH, which is suggested to improve performance via at least two mechanisms: 1) less inhibition of muscle enzymes and 2) reduced opening of muscle K_ATP channels with less ensuing reduction in excitability. Aiming at determining whether the ergogenic effect of HCO₃^- is related to effects on muscles we examined the effect of elevating extracellular HCO₃^- from 25 to 40 mM (pH from 7.4 to 7.6) on fatigue, intracellular pH (pH_i) and K⁺ efflux in isolated rat skeletal muscles contracting isometrically. Fatigue induced by 30 Hz stimulation at 30 and 37 °C was similar between soleus musles incubated in high and normal HCO₃^- concentrations. In extensor digitorum longus muscles stimulated at 60 Hz, elevated HCO₃^- did not affect fatigue at 30 °C. In solues muscles, 30 Hz stimulation induced a ~0.2 unit reduction in pH_i, as determined by using the pH-sensitive probe, BCECF. This reduction in pHi was not affected by elevated HCO₃^-. Estimation of K⁺ efflux using ⁸⁶Rb⁺ showed that elevated HCO₃^- did not affect K⁺ efflux at rest or during contractions. Similarly, other modifications of the intra- and extracellular pH had little effect on K⁺ efflux during contraction. In conclusion elevated extracellular HCO₃^- had no significant effect on muscle fatigue, pH_i and K⁺ efflux. These findings indicate that alternative mechanisms must be considered for the ergogenic effect of HCO₃^- observed in integral exercise studies.