Stimulation of an amiloride-sensitive Na+ influx pathway, which mediates Na+/H+ exchange, has been postulated to be an important step in the initiation of DNA synthesis in quiescent human fibroblasts. If the elevation of intracellular Na+ or the alkalinization of intracellular pH resulting from the activation of this system i s a trigger for subsequent mitogenic events, then its inactivation may also be important to cellular functions. We investigated the duration of the activation of Na' influx by serum in human foreskin fibroblasts (HSWP). It was found that activation of N a + influx by 10% serum was transient, declining with a tl/, = 15 min. Similarly, the Na+ content of the cells rose rapidly following serum addition and decreased with a tl/, = 15 min. In addition, both the lys-bradykinin-and the vasopressin-stimulated Nat influx and Na+ content declined with a tqh of approximately 15 min. Similar results were obtained using both Tris-buffered and Hepes-buffered, amino-acid-free EMEM. Finally, the above experiments were repeated under conditions normally used to assess the mitogenic response of cells. It was found that in cells arrested in Go by serum deprivation in C02-buffered EMEM, the serum activated Na+ flux was also transient with a tM of approximately 20 min. The desensitization of cells to serum could be readily (tn = 20') reversed by a subsequent incubation of cells in serum-free medium. Stimulation of Na+ influx by both the divalent cation ionophore A23187 and the phospholipase activator melittin in also desensitized rapidly, suggesting the process i s independent of receptor downregulation. T h e desensitization during serum preincubation occurred in both low Na+ and low pH medium suggesting that the process is not due to negative feedback on the transport system via a rise in cellular Na+ concentration or a rise in intracellular pH. Although the mechanism of desensitization is at present not known, it is likely to be a physiologically important event.