The effects of independently varying intracellular and extracellular pH on sodium pump fluxes were studied in the squid giant axon. By means of intracellular dialysis, we found that changes of intracellular pH (pHi), but not of extracellular pH, affected ouabain-sensitive Na+ efflux and K+ influx over the pH range of 6.0-8.6. Both fluxes were maximum at a pHi of 7.2-7.4. Variations away from this optimal pHi in either the acidic or alkaline direction resulted in a graded inhibition of both ouabain-sensitive fluxes. The kinetic basis for the inhibitory effect of acidic pHi was examined by comparing the kinetic parameters of activation of ouabain-sensitive sodium efflux by intracellular Na+ (Na+i) and extracellular K+ (K+o) at normal pHi with those at acidic pHi. We found that the inhibitory effect of intracellular acidity results from a reversible decrease in maximum velocity (Vmax), without an effect on the activation parameters for Na+i (K1/2 Na+i) or K+o (K1/2 K+o).