Basolateral membranes of Aplysia californica foregut epithelia contain an ATP-dependent Na /K transporter (Na /K pump or Na /K -ATPase). This Na /K pump accounts for both the intracellular Na electrochemical potential (μ̄) being less than the extracelluar Na and the intracellular K being more than the extracellular K . Also, K channel activity resides in both luminal and basolateral membranes of the Aplysia foregut epithelial cells. Increased activity of the Na /K pump, coupled to luminal and basolateral membrane depolarization altered the K transport energetics across the basolateral membrane to a greater extent than the alteration in K transport energetics across the luminal membrane. These results suggest that K transport, either into or out of the Aplysia foregut epithelial cells, is rate-limiting at the basolateral membrane.