Sodium-calcium exchange current was recorded under voltage clamp in giant excised sarcolemmal patches from adult guinea pig ventricular myocytes. An outward sodium-calcium exchange current was activated by a step increase in cytoplasmic sodium. The sodium-activated current showed a sigmoid dependence on cytoplasmic sodium concentration and a biphasic dependence on cytoplasmic calcium concentration, with peak current amplitude at 1 {dollar}\mu{dollar}M calcium. The sodium-activated current was inhibited by lanthanum, nickel, amiloride, and protons. These characteristics agree with measurements of sodium-calcium exchange in other preparations. The data suggested that block by lanthanum and cobalt is enhanced by membrane depolarization. The sodium-calcium exchange current was highly sensitive to cytoplasmic pH above 6.0. At 6.0 it was completely blocked. Proton block was not relieved by increased cytoplasmic sodium, but did appear to be relieved by increased cytoplasmic calcium, suggesting that protons compete at a calcium binding site. The potency of proton block was not measurably reduced by a temperature drop from 34{dollar}\sp\circ{dollar}C to 22{dollar}\sp\circ{dollar}C, as if proton interaction with the sodium-calcium exchanger involves a simple physical reaction. Partial proteolysis of the sodium-calcium exchanger with {dollar}\alpha{dollar}-chymotrypsin reduced its sensitivity to proton block. Proton block develops in two phases: the first phase was complete in less than one second and occurred in the absence of cytoplasmic sodium, and the second phase developed with an average half time of five seconds and did not occur in the absence of cytoplasmic sodium. Proton inhibition of sodium-calcium exchange was modeled as a series of first- and second-order reactions, such that the sodium-bound form of the sodium-calcium exchanger has a higher affinity for protons than the sodium-free form. This model reproduced the observed sensitivity of proton block of sodium-calcium exchange current to cytoplasmic sodium. In conclusion, protons inhibit the cardiac sodium-calcium exchanger by multiple mechanisms, at least one of which may involve competition at a calcium binding site. Proton block is potentiated by increased cytoplasmic sodium, which means that a rise in cytoplasmic sodium concentration will cause a fast activation of sodium-calcium exchange followed by a slow development of proton block, even if pH remains constant.