1. The inhibition of acid extrusion via Na(+)-H+ exchange caused by reducing pHo (extracellular pH) was examined in the sheep cardiac Purkinje fibre. Intracellular pH (pHi) and intracellular Na+ activity (alpha 1 Na) were recorded using ion-selective microelectrodes. Acid extrusion via Na(+)-H+ exchange was estimated from the pHi recovery rate (multiplied by intracellular buffering power, beta) in response to an internal acid load induced by 20 mM-NH4Cl removal (nominally CO2-HCO3-free media). 2. At a given pHi, acid extrusion decreased sigmoidally with decreases of pHo in the range 8.5 to 6.5 (50% inhibition of efflux occurred at a pHo between 7.0 and 7.5). This inhibition was associated with a parallel decrease in Na+ influx as evidenced from a decrease in the rise of alpha i Na measured during acid extrusion, suggesting inhibition of Na(+)-H+ exchange. 3. The background acid-loading rate (estimated by adding 1 mM-amiloride to inhibit Na(+)-H+ exchange and recording the initial rate of fall of pHi) was found to be unaffected in the steady state by changes of pHo. We therefore conclude that the slowing of pHi recovery at low pHo is due to direct inhibition of Na(+)-H+ exchange rather than to an increase of background acid loading. 4. Reducing pHo (constant pHi) inhibited acid efflux by producing a parallel shift of the efflux versus pHi relationship to lower values of pHi, consistent with a decrease in the apparent internal H+ ion affinity (pKi) of the system. 5. Raising pHi (constant pHo) also inhibited acid efflux, but this was associated with a rise in the pHo required for 50% maximal inhibition of acid efflux (pKo), consistent with an increase in apparent affinity for external H ions. Thus reduction of pHo reduces pKi (point 4) while reduction of pHi reduces pKo (point 5). 6. Inhibition by elevated Ho+ was not linearly related to the decrease in chemical driving force for Na(+)-H+ exchange, nor was it related to a reversal of the transmembrane H+ gradient. We found that efflux still occurred when pHo less than pHi. 7. Efflux was not a unique function of the transmembrane H+ ratio (i.e. pHo-pHi). At appropriate values of pHi and pHo, acid efflux could be kept constant despite a four-fold change in the transmembrane H+ ratio. 8. Inhibition by low pHo was a saturating function of Ho+ ions with a Hill coefficient of 1.2.(ABSTRACT TRUNCATED AT 400 WORDS)