The aim of this study was to determine the effects of 3 wk of daily rinsing with amine fluoride/stannous fluoride (AmF/SnF(2)) mouthrinse on plaque formation at buccal and interproximal sites, and on the acid production in plaque, in a randomized clinical trial with 30 participants. The amount of plaque was scored according to Turesky's modification of the Quigley and Hein index. Plaque samples were collected, before and after sucrose rinsing, from the buccal and interproximal surfaces of upper (pre)molars at two baseline visits and on days 2 and 7 after the discontinuation of 3 wk of daily rinsing. Metabolic acid ions were determined by capillary electrophoresis. The results at baseline showed higher lactic acid concentrations in resting interproximal plaque than in buccal plaque, and a higher acid production in response to sucrose challenge in buccal plaque than in interproximal plaque. After 3 wk of use of the AmF/SnF(2) mouthrinse, no significant differences in plaque scores were observed, and the alleged reduction in acidogenicity of dental plaque was not significant on the second day after the last mouthrinse. We conclude that 3 wk of use of AmF/SnF(2) rinse once daily does not result in a reduction of plaque formation or in a reduction of sucrose metabolism in buccal and interproximal plaque after discontinuing the rinse.