We assessed the in vivo effects of zinc and chlorhexidine (CH) on plaque ureolysis and glycolysis in five volunteers. We monitored plaque pH by a surface glass electrode on two teeth in each subject, after topical application of either 5% wt/vol urea or 5% wt/vol glucose solutions. The recordings were repeated 15 and 75 min after a single mouthrinse, with either 20 mmol/L zinc acetate or 0.33 mmol/L CH diacetate. Ureolytic activity decreased significantly (p less than 0.03) up to 75 min after a single mouthrinse with the zinc-containing test solution. CH had no effect on plaque ureolytic activity. Acid production by dental plaque decreased significantly (p less than 0.01) up to 75 min after a single mouthrinse with either the zinc- or the CH-containing test solution.