The aim was to study the short-term effect of a varying sucrose exposure on plaque acidogenicity and cariogenic microflora in two groups of subjects with different levels of mutans streptococci (MS). Eight subjects with low (<10(4)) and 8 with high (>10(6)) numbers of MS per millilitre saliva participated. Three 7-day test periods were conducted. During two of these, the subjects rinsed either 5 or 10 times daily with 10% sucrose; the third period without any mouth rinses served as control. The subjects refrained from oral hygiene during the last 3 days of each test period. On day 7, the following parameters were measured: plaque pH after a sucrose rinse, numbers of MS, Streptococcus sanguis and lactobacilli in saliva, percent MS in plaque and plaque index. The results revealed that plaque acidogenicity was more pronounced for the high-MS group compared to the low-MS group after all three test periods, i.e. lower resting pH, deeper pH falls and a lower final pH. For both groups, the greatest pH-lowering capacity of plaque was found after the period with 10 sucrose rinses/day. An increase in bacterial counts was noted for both groups during the test periods with the 5- and 10-time rinse regimen; this increase was larger for the high-MS group compared to the group with low MS counts. The highest plaque index was, irrespective of the test period, found for the high-MS group.