Gingival crevice and periodontal pocket pH, measured directly with glass micro-electrodes, was near neutral at most sites in most individuals (mean pH 6.92 +/- 0.03 SEM, 69 subjects). Periodontal state ranged from healthy to periodontitis but neither clinical evidence of gingivitis at a site nor pocket depth were associated with crevicular pH different from that at healthy sites. This finding contradicts earlier reports that gingivitis is associated with a crevicular pH as alkaline as pH 9.06. Metallic antimony electrodes as used by earlier investigators were found to give pH readings that were too high by as much as 1.5 pH units in the presence of organic reducing agents of the type produced by oral bacteria within gingival crevices. In contrast, glass micro-electrodes respond only to hydrogen ions and thereby provided accurate measurements of pH even in the presence of organic reducing agents. Loss of CO2 to the atmosphere from biological fluids that are bicarbonate buffered resulted in a shift to alkaline pH by as much as 1 pH unit. As a result, only measurements taken within gingival crevices or periodontal pockets can provide accurate measurements of crevice or pocket pH.