Fish exposed to elevated water CO(2) experience a rapid increase in blood CO(2) levels (hypercapnia), resulting in acidification of both intra- and extra-cellular compartments. While the mechanisms associated with extracellular pH regulation have been well explored, much less is known about intracellular pH (pH(i)) regulation. There is great interest in developing non-animal models for research. One such model is the rainbow trout hepatoma cell line (RTH 149), which has been used to study a wide range of topics; however, no studies have investigated its potential use in pH(i) regulation. Employing the pH-sensitive fluoroprobe BCECF, the present study examined pH(i) regulation in RTH 149 under normocapnia and during extracellular acidification induced by either elevated CO(2) or 1 M HCl. During exposure to hypercapnia, RTH 149 cells were acidified without recovery as long as the elevated CO(2) was maintained. In addition, rates of pH(i) recovery from NH(4)Cl-induced acidosis were significantly lower in cells exposed to hypercapnia or HCl compared to that in normocapnic cells, indicating that elevated CO(2) indirectly impeded pH(i) recovery through a reduction in pH(e) and/or pH(i). Moreover, pH(i) regulation in RTH 149 was EIPA-sensitive, suggesting that an NHE may be involved. Overall, RTH 149 may have the potential for identifying transporters likely to play a role in pH(i) regulation in fish. However, it should not be used as a complete replacement for in vivo studies, especially to quantify acid-base regulatory ability at whole animal level, since RTH 149 appeared to have enhanced pH(i) recovery rates relative to primary hepatocytes.