A system employing an Ingold glass electrode was shown to give reliable measurements of pH drops in dental plaquein situ(Stephan curve readings). The system was used to demonstrate that mouthrinses of 0.2 per cent aqueous solutions of stannous fluoride reduced the pH drops markedly for at least seven hours. A reduction of the stannous fluoride concentration increased the pH drops and decreased the duration of the inhibiting effect. A commercial toothpaste containing stannous fluoride and stannous pyrophosphate had an effect similar to the 0.2 per cent mouthrinse.It was shown that tin accumulated in dental plaque after application of solutions containing stannous fluoride. About 40 per cent of the amount of tin present in the plaque immediately after the mouthrinse was still retained seven hours later.It is suggested that the reduction in acid formation may be caused by stannous ions adsorbing to the bacterial cell wall thus disturbing membrane transport mechanisms, or through inhibition of enzyme systems essential in the fermentation of sugars.The observed effect may be a part of the mechanism involved in the caries preventive function of stannous fluoride.