Subjects using 30 mM zinc phenolsulphonate as a mouthwash retained 12 per cent of the zinc. Salivary zinc concentration was increased by using mouthwashes containing 17-35 mM zinc as the sulphate, phenolsulphonate or citrate. For 17 mM zinc sulphate or phenolsulphonate, the effect lasted 3-4 h. Zinc retained in the mouth gave visible fluorescence after rinsing with 8-hydroxyquinoline and was particularly evident on the tongue, cheek mucosa and dental plaque. The concentration of zinc in plaque was increased 13-19-fold 1 h after using 31 or 18 mM zinc phenolsulphonate. A 3-fold increase was still present 6 h later for the 31 mM mouthwash. Zinc salts inhibited acid production from [14C]-glucose in vitro by plaque at concentrations which were found in plaque in vivo after using the mouthwashes. The effect of zinc on the metabolic activity of plaque may reduce the growth rate of plaque-bacteria and so decrease plaque growth.