an in situ evaluation of the potential rehardening effect of fluoridated and non-fluoridated toothpastes with or without air polishing was conducted. ten volunteers, using acrylic palatal appliances containing two bovine enamel blocks with artificial carious lesions, took part in this study. four times a day, after the main meals and at night, the volunteers, in a habitual way, brushed their natural teeth with the dentifrice indicated to the experimental design and after that the appliances were put again into the mouth. they were divided into 4 different groups: g1 - control - non-fluoridated dentifrice; g2 - fluoridated dentifrice; g3 - non-fluoridated dentifrice, but having a previous prophylaxis using air polishing; g4 - fluoridated dentifrice and previous air polishing. the effects of treatments on enamel rehardening were evaluated in the blocks that were assessed by surface microhardness, and the percentage of surface microhardness change (%reh) was calculated in relation to the baseline values. the results showed that %reh was higher in the groups with fluoridated dentifrice, and professional prophylaxis did not have an additional effect in the groups of fluoridated dentifrices (p<0.05). the data suggested that, in the absence of fluoride, removal of dental plaque helped to increase the process of enamel rehardening.