An amiloride-inhibitable, Na(+)-H+ antiporter was identified in the basolateral membrane of turtle colon by measuring 22Na+ fluxes across isolated tissues apically permeabilized with the pore-forming antibiotic amphotericin B. In cells shrunken by exposure to Cl(-)-free (gluconate) solutions and treated with ouabain to block the Na-K-ATPase, Na+ movement across the basolateral membrane was due entirely to the antiporter. Elevation of cytosolic Na+ was associated with an amiloride-inhibitable outward current across the basolateral membrane. The sensitivity of the current to various amiloride analogues paralleled that of Na+ exchange rather than that of the apical Na+ channel. Furthermore, cell volume changes altered basolateral Na+ exchange and basolateral Na+ conductance in a parallel fashion. We propose that this amiloride-sensitive basolateral Na+ conductance represents an altered operating mode of a basolateral Na(+)-H+ exchanger.