OBJECTIVE To determine the comparative effects of sodium (NaF) and amine fluoride dentifrices, as well as the effect of different silica abrasives used in NaF dentifrices, on caries formation in rats. METHODS Harlan Sprague Dawley mixed sex, weanling, littermate rats were fed a cariogenic diet and inoculated with S. sobrinus strain 6715 to induce caries formation. The dentifrices evaluated contained 0.31% NaF with precipitated silica I as the abrasive system; 0.31% NaF with sodium bicarbonate and hydrated silica; placebo; amine fluoride (0.14%) containing silica (Elmex); 0.31% NaF with precipitated silica II (Sensodyne F); and 0.31% NaF with hydrated silica. Dentifrice slurries were administered 7 days per week for 3 weeks at which time the animals were euthanized and the hemijaws removed, coded and stained with a murexide solution. The recovery of S. sobrinus after the 3-week study clearly showed that all of the six groups were infected with S. sobrinus. The Keyes scoring method was then used to assess the caries formation within the six groups. RESULTS Caries scores showed that all five experimental fluoride dentifrices were equally efficacious statistically in reducing caries formation when compared to the placebo with reductions ranging from 26 to 43%.