The plaque sampling method and wire telemetry using miniature, glass pH electrodes on the buccal surface of mandibular first permanent molars, compared human dental plaque pH responses to a 10 per cent solution of sucrose. Nine subjects abstained from oral hygiene and were tested in morning sessions on 5 consecutive days. The mean resting plaque pH value for all sessions by plaque sampling was 6.60 +/- 0.18 (mean +/- SD) and by telemetry was 6.56 +/- 0.19. After a 1 min sucrose rinse, 1-day-old plaque showed a decrease in pH approx. 5.5 by both methods. The 2-, 3-, 4- and 5-day-old plaque, the mean minimum pH achieved was 4.69 +/- 0.28 as measured by sampling and 3.94 +/- 0.49 as determined by telemetry. The times taken to reach minimum pH as determined by the two methods were almost identical. The electrode calibration data showed that all the telemetry electrodes responded consistently for all 5 days of study. Scanning electron microscopy and microbiological analysis of electrode tips and enamel replicas revealed that plaque accumulated on glass at the same rate with a similar bacterial composition to plaque formed on clean enamel. It is proposed that glass microelectrodes and wire telemetry are a reasonable means for continuously monitoring plaque pH in situ. Comparison with plaque sampling suggested that telemetric responses reflect the type of plaque which accumulates at particular sites on the dentition.