The aim of the present study was to evaluate the cariogenicity of cows', human and infant formula milks, supplemented or not with fluoride, in rats. Sixty female Wistar rats were desalivated and infected with Streptococcus sobrinus 6715.Animals were divided into six groups: group 1, sterilised deionised distilled water (SDW; negative control); group 2, 5 % sucrose added to SDW (positive control); group 3, human milk; group 4, cows' milk; group 5, Ninho® formula reconstituted with SDW; group 6, Ninho® formula reconstituted with 10 parts per million F and SDW. At day 21 the animals were killed and their jaws removed to quantify total cultivable microbiota, Strep. sobrinus and dental caries. The concentration of carbohydrate and fluoride in the milks was analysed. The Kruskal–Wallis test (α = 5 %) was used to analyse the data. The caries score by the milk formula was as high as that provoked by sucrose. Regarding smooth-surface caries, human milk was statistically more cariogenic than cows' milk, which did not differ from the SDW and the Ninho® with fluoride (P>0·05). Groups 2–6 showed higher Strep. sobrinus counts when compared with the negative control group (P < 0·05) but no statistically significant difference was found among them (P>0·05). HPLC analysis showed that infant formula had 9·3 % sucrose and 3·6 % reducing sugars. The infant formula should be considered cariogenic due to the sugars found in it, but fluoride supplementation reduced its cariogenic effect. The human milk was more cariogenic than the cows' milk but not as much as the formula milk.