The effects of 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS), a potent anion transport blocker, on transmembrane action potentials (APs) and the sodium current (INa) of guinea pig ventricular myocytes were examined by using conventional microelectrode and whole-cell patch-clamp recording techniques. In papillary muscle preparations, DIDS (≥0.1 mM) suppressed the maximal upstroke velocity (v˙max) of the AP without significant changes in other AP parameters. Extracellular application of DIDS on single cardiomyocytes isolated from the guinea pig ventricle markedly reduced the peak amplitude of the tetrodotoxin (TTX)-sensitive and voltage-activated sodium current. The concentration-dependent block of DIDS could be expressed by the Hill equation with a Hill coefficient of 0.97 and a dissociation constant of 0.15 mM at a holding potential of (VH) −120 mV. DIDS (0.1 mM) shifted the steady-state inactivation curve for INa toward more negative potentials by 6.0 ± 0.5 mV and the activation curve to more positive potentials by 5.0 ± 1.0 mV, although the slope factors were unaffected. With repetitive depolarizing pulses from −120 mV, DIDS produced a use-dependent block on the INa. Recovery of INa from inactivation was slowed (time constant = 245 ms, compared with 10 ms of control) in the presence of 0.1 mM DIDS. In the two-pulse experiments, DIDS produced two distinct phases of development of INa block, the rapid phase (τ = 5 ms) caused by an open channel block, and the slower phase (τ = 382 ms) induced by an inactivated channel block. These results suggest that the Cl− transport blocker DIDS has a direct inhibitory effect on the cardiac sodium channel. DIDS-induced use dependence of INa block may result from the interaction of the drug with sodium channels in both the open and inactivated channel states.