Objective: To determine if the in vitro 10-day pH-cycling model used for permanent teeth could be utilized to evaluate de/remineralization effects, on the enamel of primary teeth, of child formula toothpastes. Methods: Sound extracted primary anterior teeth were coated with nail-varnish, leaving 1 mm wide window prior to being placed in a demineralizing solution to produce artificial carious lesions in enamel. The teeth were subsequently cut longitudinally into 100 µm thick sections. The 72 specimens were randomly assigned to 6 groups. In Set 1 contained the specimens in Group a, which were treated with a pea-sized (0.32g) quantity of non-fluoride toothpaste (First Teeth®), and Groups b and c which were treated with half (0.16g) and pea-sized quantities of fluoridated toothpaste (Colgate®), and cycled for 10 days. The specimens in Set 2 (Groups A, B and C) were similarly treated but subjected to 7-day pH-cycling. Each pH cycle involved 3 hours of demineralization twice a day with 2 hours remineralization in between. One minute treatment with dentifrice slurry, 3:1 deionized wate/toothpaste (5ml/section) was given before the first demineralising cycling and both before and after the second demineralizing cycle. Sections were placed in remineralizing solution overnight. Results: After day 8 the lesions in Set 1 extended into dentine and so could not be evaluated. Polarized light microscopy and microradiography were used to evaluate the lesions subjected to 7-day pH-cycling. Lesions in Groups A and B increased in depth and area by approximately 50%, while those in Group C increased in depth and area by 20%; however, no statistically significant differences occurred between the groups. Conclusions: If this model pH cycling is to be used for primary teeth, a reduction from 10 to 7 days in length should be considered.