1. The influence of sarcolemmal Na+-H+ exchange upon intracellular Na+ activity (aiNa), intracellular pH (pHi), extracellular surface pH (pHs) and tonic tension was investigated in sheep cardiac Purkinje fibres. Intracellular ion activities were measured with liquid sensor ion-selective micro-electrodes. A two-micro-electrode voltage-clamp was also used to control membrane potential while simultaneously recording tonic tension. 2. Inhibition of the sarcolemmal Na+-K+ pump by strophanthidin (10 mumol/l) produced a rise in aiNa, an increase in [Ca2+]i as evidenced by a rise in tonic tension, and a fall in pHi of 0.1-0.3 units. The intracellular acidosis has been shown previously to be linked to the rise in [Ca2+]i (Vaughan-Jones, Lederer & Eisner, 1983). 3. Amiloride (1-2 mmol/l), an inhibitor of Na+-H+ exchange, produced a small reversible decrease in pHi and aiNa. Both effects became more pronounced in strophanthidin-exposed fibres. In addition, pHi decreased during application of strophanthidin and this decrease was reversibly inhibited by amiloride. It is concluded that sarcolemmal Na+-H+ exchange is stimulated following inhibition of the Na+-K+ pump. 4. In strophanthidin-exposed fibres, a rise in [Ca2+]i resulted in an intracellular acidosis which could still be observed in the presence of amiloride (1 mmol/l). This suggests that the fall in pHi was not caused by a modulatory effect of [Ca2+]i on sarcolemmal Na+-H+ exchange. 5. Tetrodotoxin (TTX) produced a small fall in aiNa (ca. 0.5 mmol/l) which was not augmented in the presence of strophanthidin. Furthermore, the effects on aiNa of TTX and amiloride were additive. Thus the influence of amiloride on aiNa does not involve blockade of voltage-gated Na+ channels. 6. The stoicheiometry of Na+-H+ exchange, estimated from the rates of change of pHi and aiNa in amiloride, appeared to be electroneutral (1:1). The stoicheiometry was unaffected by changes in pHi. 7. In strophanthidin-exposed fibres (i.e. aiNa is elevated), the recovery of pHi from an intracellular acidosis (brought about by brief exposure to NH4Cl) was slowed greatly by amiloride (1-2 mmol/l). The rise in aiNa that occurred during pHi recovery was also reduced by amiloride. It is concluded that Na+-H+ exchange can be stimulated by a fall in pHi under conditions where aiNa is elevated. However, at a given pHi, its rate of recovery was slower in the presence than in the absence of strophanthidin.(ABSTRACT TRUNCATED AT 400 WORDS)