Ion-sensitive field-effect transistor (ISFET) pH electrodes were used to monitor changes in plaque pH at the base of artificial occlusal surface fissures and at interproximal sites. Bovine enamel was used to construct fissures (1.5 x 0.1 x 1.0 mm) containing a small ISFET electrode. The fissures were fixed to carrier appliances and worn by 4 human volunteers. After plaque accumulation for 4 days changes in pH were monitored by wire telemetry following 1-min rinses with 10% solutions of either sorbitol or sucrose. Results were compared to data obtained from interproximal sites in the same subjects. Responses to sorbitol in the fissure and on the proximal surfaces were minimal and showed no significant difference in minimum pH (5.9 +/- 0.4 and 6.1 +/- 0.3, respectively) and area under pH 7.0. The response to sucrose at the two sites was very different revealing unique pH profiles which were statistically significantly different with regard to minimum pH (5.0 +/- 0.3, fissure and 4.3 +/- 0.2, proximal) and area under pH 5.7. Thus, the acidogenic potential of fermentable carbohydrate at two caries-prone sites in the human dentition is significantly different and conclusions based on interproximal telemetry measurements may not be applicable to occlusal surface fissures.