One of the earliest responses of quiescent mammalian cells to the addition of serum is an increase in intracellular pH (pHin). This pHin change is generally believed to be due to an increased activity of Na+/H+ exchange. A number of investigators have observed steady-state differences in pHin between cells in the presence and absence of serum. However, no one has examined differences in pHin regulation that may exist between cells chronically exposed to, or deprived of serum. In this study, we investigated the effects of serum deprivation to identify those components of pHin regulation that were associated with quiescence. To do this, we examined pHin in cells growing chronically in 10% serum as well as in cells that were either acutely (1.5-2 hr) or chronically (48 hr) deprived of serum. Intracellular pH was monitored using the fluorescence of intracellularly loaded pyranine dye. Our results indicate that the resting pHin values of chronically or acutely serum-deprived cells were not significantly different from each other yet, in both cases, were lower than those observed in cells exposed to 10% serum. Furthermore, we observed significant increases in pHin of both acutely or chronically serum-deprived cells in response to the addition of serum at various concentrations, in the presence of 24 mM bicarbonate. Chronically serum-deprived cells had slightly smaller responses and were more sensitive to lower concentrations of serum than were acutely deprived cells. Therefore, our data suggest that long-term serum deprivation affects the magnitude and sensitivity of pHin to serum stimulation and causes the loss of some form of pHin regulatory mechanism(s).