Effects of long-term, subtotal inhibition of Na+-K+ transport, either by growth of cells in sublethal concentrations of ouabain or in low-K+ medium, are described for HeLa cells. After prolonged growth in 2 X 10(-8) M ouabain, the total number of ouabain molecules bound per cell increases by as much as a factor of three, mostly due to internalization of the drug. There is only about a 20% increase in ouabain-binding sites on the plasma membrane, representing a modest induction of Na+, K+-ATPase. In contrast, after long-term growth in low K+ there can be a twofold or greater increase in ouabain binding per cell, and in this case the additional sites are located in the plasma membrane. The increase is reversible. To assess the corresponding transport changes, we have separately estimated the contributions of increased intracellular [Na+] and of transport capacity (number of transport sites) to transport regulation. During both induction and reversal, short-term regulation is achieved primarily by changes in [Na+]i. More slowly, long-term regulation is achieved by changes in the number of functional transporters in the plasma membrane as assessed by ouabain binding Vmax for transport, and specific phosphorylation. Parallel exposure of cryptic Na+,K+-ATPase activity with sodium dodecyl sulfate in the plasma membranes of both induced and control cells showed that the induction cannot be accounted for by an exposure of preexisting Na+,K+-ATPase in the plasma membrane. Analysis of the kinetics of reversal indicates that it may be due to a post-translational event.