The object was to find out whether site-specific variations of pH might correlate with those of oral clearance as indicated by past studies using fluoride, sucrose and glucose as markers. Eleven subjects with 24 h growth of dental plaque rinsed the mouth with a 10% solution of sucrose. The pH of plaque, harvested from the labial or buccal cervical and approximal regions of certain teeth or groups of teeth, was then measured. The results confirmed previous reports that plaque pH varies from site to site in the mouth; the pattern observed was similar in each of the subjects. In the anterior part of the mouth, concentrations of hydrogen ion were invariably higher in plaque from the upper than from the lower surfaces of the teeth. The reverse pattern was found in the posterior part. This site-specific pattern was similar to that of oral clearance, as indicated by comparison with known values for glucose concentrations, a finding consistent with the view that site-specific pattern of plaque pH may reflect or at least be influenced by such differential clearance patterns.