We measured bicarbonate, ammonia, and luminal pH in segments of the rabbit outer medullary collecting duct (OMCD) to determine the relationship between luminal pH and ammonia transport. Both the inner-stripe and outer-stripe portions of the OMCD absorbed bicarbonate at high rates. The outer stripe OMCD generated an acidic pH disequilibrium that was reversibly dissipated by exogenous luminal carbonic anhydrase. In contrast, the inner stripe OMCD did not generate a spontaneous pH disequilibrium unless perfused with the carbonic anhydrase inhibitor acetazolamide. Ammonia secretion was three times more rapid in the outer stripe OMCD than in the inner stripe OMCD. We conclude the following. 1) Both the inner-stripe and outer-stripe portions of the rabbit OMCD secrete protons at substantial rates. 2) Functional luminal carbonic anhydrase is present in the inner stripe OMCD but absent from the outer stripe OMCD. 3) Ammonia secretion occurs predominantly by NH3 diffusion in both portions. 4) The luminal pH disequilibrium, which is normally present in the outer stripe OMCD, enhances ammonia secretion.