We have made continuous pH measurements of plaque grown between teeth in the Orofax artificial mouth system (Bibby, B.G. and Huang, C.T., J Dent Res, In press). For the present tests, two half-teeth were mounted in wax to form an approximal space, through one tooth, a miniature glass electrodes was set flush with the enamel surface. It was connected through a field-effect transistor amplifier (FET) to a millivolt/pH metert and paper chart recorder using a small flexible Ag/AgCl reference electrode.0 The pH electrode was calibrated in phosphate buffer at pH 7 and pH 4. To grow plaque, the interproximal area was inoculated with a sonicated suspension of human plaque; it was then placed under the flow of saliva and re-inoculated four and eight h later. On the second day plaque development was enhanced by adding a 35-mg sucrose-corn starch tablet three times at intervals of four h. Plaque three to eight days old was tested. Between tests and each night the plaque was kept in the flow of saliva. The electrode calibration was checked daily. Most tests were made by stopping the flow of saliva and adding 0.1 ml of a 2% solution of a sugar, starch, sorbitol, or xylitol to the approximal area. Thereafter a drop of saliva, water, or substrate was added every 30 min to prevent drying, and the pH was observed until a pH minimum was reached. If tests were made while saliva continued to drop, the pH depressions were smaller and of shorter duration. To restore the pH to the starting level, 0.1 ml of 1% carbamide was added, and the flow of saliva was allowed to run for 60 min to wash out excess substrate and carbamide. Very little acid production was evident with sorbitol addition. Lactose gave a pronounced pH drop, reaching a pH minimum of 4.4 after 65 min. Sucrose and several starches gave closely comparable results. The pH decreases were more rapid than with lactose and reached a stable level of pH 3.5 after 50 min. It is interesting that potato starch, which had been partly hydrolyzed by boiling, is as acidogenic as sucrose in this system. Gram-stained smears on the third and eighth days showed a mainly gram-positive coccal flora with some gram-negative and sarcinal types,