B icarbonate may be secreted into the intestinal lumen in cholera because: HCO₃⁻ ions are transported, or because OH⁻ ions accumulate and react with dissolved CO₂ to form HCO₃⁻. If HCO₃⁻ ions are transported into the lumen from the interstitial fluid, lumenal P_CO2 should increase (HCO₃⁻ ⇌ OH⁻ + CO₂); if OH⁻ accumulates, P_CO2 should diminish. Net movement of H₂O, and HCO₃⁻, and changes in pH and P_CO2 in lumenal fluid were studied in adjacent segments of rabbit ileum in vivo, one of which was exposed to choleragen. 4 h after exposure, segments were drained and infused with gassed Krebs-Henseleit solution whose P_CO2 exceeded arterial P_CO2. After 45 min, fluid was collected anaerobically from control and cholera segments. Among 13 cholera segments, lumenal P_CO2 diminished by a mean of 8.4 torr and was less than femoral arterial blood in six instances. In the paired control segments, mean P_CO2 increased by 4.4 torr, and was always greater than arterial P_CO2. Dilution could not account for the low P_CO2 in cholera segments because in hypertonic solutions that caused water to move into the lumen, the P_CO2 did not differ from control values obtained with isotonic solutions. The results suggest that OH⁻ accumulation (by addition of OH⁻ or removal of H⁺) causes HCO₃⁻ secretion in cholera. This does not result from secretion of some other base (e.g., HPO₄⁻), because HCO₃⁻ accounts for most of the base in the lumenal fluid. The P_CO2 changes suggest that OH⁻ reacts with CO₂ at the cell-lumen interface, but reaction at the cell-interstitial fluid interface cannot be excluded.