We examined the relationship among intracellular sodium activity (aiNa), intracellular pH (pHi), and delayed afterdepolarization (DAD) in guinea pig ventricular papillary muscle fibers exposed to K-free solution in vitro, using Na+- and H+-selective microelectrodes. In unstimulated papillary muscles, exposure to K-free solution caused a rapid increase in aiNa of 16.2-22 mM at the end of a 20-min superfusion period with rates of 0.51-0.73 mM/min. This was paralleled by a decrease in intracellular pH of 0.10-0.15 units. DADs were induced after a train of driven action potentials 5-15 min after superfusion with K-free solution at the elevated aiNa from the steady-state level. Prevention of intracellular acidosis associated with the increased aiNa in K-free solution by external alkalosis at pH 8.0 enhanced the amplitude of the DAD, whereas restoration of external pH to 7.4 and a further acidification of external pH to 6.4 suppressed DADs. We concluded that intracellular acidosis associated with the increased aiNa per se increased the likelihood for inhibition of sarcoplasmic reticulum Ca2+ release in spite of increased cytosolic Ca2+.