The effect of the wide natural variation in oral ureolysis rates on the pH changes resulting from simultaneous metabolism of 25 mM urea and 2.8 mM glucose in salivary-sediment bacteria were investigated. The pH curves were complex, and included distinctive plateaux indicative of balanced acid and base production. These neutralization plateaux occurred at different pHs, which were a function (r2 = 0.98) of the ureolytic rate as measured by the log of the initial pH-change rate in the urea-only reaction. In the simplest case, the pH curve was characterized by a rise or fall to the neutralization plateau, a variable period of time at the plateau (up to 1 h), then a pH rise. The pattern of pH changes induced by glucose alone varied between different sediments: in some cases, the pH decreased smoothly to an end-point; in others, the curve was more complex, and these features became superimposed on the urea/glucose curve. The rate of ureolytic ammonia release was almost constant and unaffected by simultaneous carbohydrate metabolism. Concomitant metabolism of endogenous carbohydrate present in sediments prepared 1-2 h following a meal was of sufficient magnitude to affect ureolytic pH curves. If the ureolytic activity was high, this effect was negligible; if it was low, metabolism of the endogenous carbohydrates could completely suppress the ureolytic pH rise. Soluble salivary components had little effect on ureolysis but pH changes were modified by buffering, and the presence of urea, ammonia, N-catabolic and acidogenic substrates in the saliva.