Helicobacter pylori resists gastric acidity by modulating the proton-gated urea channel UreI, allowing for pH_out-dependent regulation of urea access to intrabacterial urease. We employed pH- and Ca²⁺-sensitive fluorescent dyes and confocal microscopy to determine the location, rate, and magnitude of pH changes in an H. pylori-AGS cell coculture model, comparing wild-type bacteria with nonpolar ureI-deletion strains (ureI-ve). Addition of urea at pH 5.5 to the coculture resulted first in elevation of bacterial periplasmic pH, followed by an increase of medium pH and then pH in AGS cells. No change in periplasmic pH occurred in ureI-deletion mutants, which also induced a slower increase in the pH of the medium. Pretreatment of the mutant bacteria with the detergent C₁₂E₈ before adding urea resulted in rapid elevation of bacterial cytoplasmic pH and medium pH. UreI-dependent NH₃ generation by intrabacterial urease buffers the bacterial periplasm, enabling acid resistance at the low urea concentrations found in gastric juice. Perfusion of AGS cells with urea-containing medium from coculture at pH 5.5 did not elevate pH_in or [Ca²⁺]_in, unless the conditioned medium was first neutralized to elevate the NH₃/NH₄⁺ ratio. Therefore, cellular effects of intrabacterial ammonia generation under acidic conditions are indirect and not through a type IV secretory complex. The pH_in and [Ca²⁺]_in elevation that causes the NH₃/NH₄⁺ ratio to increase after neutralization of infected gastric juice may contribute to the gastritis seen with H. pylori infection.