The combined effects of intracellular lactate and proton accumulation on cell volume, V_c, were investigated in resting Rana temporaria striated muscle fibres. Intracellular lactate, and H⁺ concentrations were simultaneously increased by exposing resting muscle fibres to extracellular solutions that contained 20-80 mM [Na-lactate]. Cellular H⁺ and lactate entry was confirmed using pH-sensitive electrodes and ¹H-NMR respectively and effects on V_c measured using confocal microscope xz-scanning. Exposure to extracellular [lactate] up to 80 mM produced significant changes in pH and intracellular lactate (from a pH of 7.24 ± 0.03, n = 8 and 4.65 ± 1.07 mM, n = 6 respectively in control fibres to 6.59 ± 0.03, n = 4 and 26.41 ± 0.92 mM, n = 3 respectively) that were comparable to those observed following fatiguing stimulation (6.30-6.70 and 18.04 ± 1.78 mM, n = 6 respectively). Yet, the increase in intracellular osmolarity expected from such an increase in intracellular lactate did not significantly alter V_c. Simulation of these experimental results modified from the charge-difference model of Fraser & Huang (2004) demonstrated that such experimental manouevres produced changes in intracellular [H⁺] and [lactate] comparable to those observed during muscle fatigue and accounted for this paradoxical conservation of V_c through balancing, negative osmotic effects resulting from the net cation efflux that would follow a titration of intracellular membrane-impermeant anions by the intracellular accumulation of protons (Fraser et al., 2005a). It demonstrated that with established physiological values for intracellular buffering capacity and the permeability ratio of lactic acid and anionic lactate, P_LacH:P_Lac- this would provide a mechanism that precisely balanced any effect on cell volume resulting from lactate accumulation during exercise.