The effects of a new, potent, and selective inhibitor of the Na(+)/Ca(2+) exchange, SEA-0400 (SEA), on steady-state outward (forward exchange), inward (reverse exchange), and Ca(2+)/Ca(2+) transport exchange modes were studied in internally dialyzed squid giant axons from both the extra- and intracellular sides. Inhibition by SEA takes place preferentially from the intracellular side of the membrane.