Fish, exposed to elevated water CO(2), experience a rapid elevation in blood CO(2) (hypercapnia), resulting in acidification of both intra- and extra-cellular compartments. White sturgeon, Acipenser transmontanus, are exceptionally CO(2) tolerant and can regulate tissue intracellular pH (pH(i)) in the presence of a pronounced hypercapnic acidosis (preferential pH(i) regulation). In this study, pH(i) regulatory capacity of sturgeon liver cells in primary culture was examined to assess the suitability of employing this in vitro system to understand in vivo CO(2) tolerance in sturgeon. Using the pH-sensitive fluoroprobe BCECF, real-time changes in resting pH(i) and rates of pH(i) recovery were investigated during exposure to hypercapnia (3 and 6% CO(2)) in the absence and presence of additional acid loads induced by (20 mM) ammonium prepulse. During short-term (10 min) exposure to hypercapnia (3 and 6% CO(2)), sturgeon cells were acidified and no pH(i) compensation was observed. However, when exposure to 6% CO(2) was extended to over 19 h, the CO(2)-induced intracellular acidosis was partially compensated by a pH(i) increase of over 0.2 pH unit despite the sustained extracellular acidosis, indicative of a capacity for preferential pH(i) regulation in vitro. Since this capacity in sturgeon liver is present both in vivo and in vitro, the transmembrane transporters involved may be the same. Therefore, cell culture may be a suitable tool to identify the transporters (i.e., the cellular mechanisms underlying in vivo CO(2) tolerance) in white sturgeon and possibly in other hypercapnia-tolerant species.