Plaque pH was studied during 60 min in situ in 10 subjects after eating various breads. The pH response was then compared to glycemic index (GI; calculated from the incremental blood glucose area) obtained from our earlier investigations. The following four products were tested: (1) barley kernel bread (BKB), (2) BKB, sourdough-fermented (BKBS), (3) white wheat bread (WWB), and (4) syrup-sweetened wheat-rye bread (SWRB). BKB was also tested with more intensive chewing and SWRB with the addition of fat. A 5% sucrose solution served as a control. The pH drops with all the breads were considerably smaller than with the sucrose solution during the first 15 min. From 30 min and onwards the breads gave similar, or even lower (SWRB) pH than sucrose. There was a great difference in pH response among the four breads, with the most pronounced pH fall for SWRB, followed by WWB (based on the AUC values). Intensive chewing of BKB increased, while the addition of fat to SWRB reduced the pH fall, in both cases by about 0.2 pH units. A high correlation (r = 0.94) between plaque pH and GI was found, i. e. the more pronounced the pH drop in plaque, the higher the GI in blood. Therefore, both from a cariogenic and from a metabolic point of view, breads with a low GI should be recommended.