To study the properties of the Na extrusion mechanism, giant muscle fibers from barnacle (Balanus nubilus) were internally perfused with solutions containing tracer ~2Na. In fibers perfused with solutions containing adenosine 5'-triphosphate (ATP) and 30 mM Na, the Na efflux into 10 mM K seawater was ~ 25-30 pmol/cm~-s; 70% of this efflux was blocked by 50-100 #M ouabain, and ~ 30% was blocked by removal of external K. The ouabainsensitive and K-dependent Na effiuxes were abolished by depletion of internal ATP and were sigmoid-shaped functions of the internal Na concentration ([Na]i), with half-maxima at [Na]i --20 raM. These sigmoid functions fit the Hill equation with Hill coefficients of ~ 3.5. Ouabain depolarized ATP-fueled fibers by 1.5-2 mV ([Na]i --30 mM) but had very little effect on the membrane potential of ATP-depleted fibers; ATP depletion itself caused a 2-2.5-mV depolarization. When fueled fibers were treated with 3,4-diaminopyridine or Ba 2+ (to reduce the K conductance and increase membrane resistance), application of ouabain produced a 4-5-mV depolarization. These results indicate that an eleetrogenic, ATP-dependent Na-K exchange pump is functional in internally perfused fibers; the internal perfusion technique provides a convenient method for performing transport studies that require good intracellular solute control.