OBJECTIVE To evaluate enamel permeability after external bleaching procedures. METHODS The coronal portion of 75 human canines were covered with an acid resistant varnish, except for a 7.1 mm2 circular area on the labial surface and, according to a randomized complete block design, specimens were allocated to bleaching treatments using carbamide peroxide-containing agents (CP): 10%, 16% or 37%; or a 35% hydrogen peroxide product (HP). The control group was exposed to artificial saliva. The bleaching agents were applied following the manufacturer's recommendations over 21 days. In between bleaching exposures, specimens were stored in artificial saliva. Samples were then prepared according to a histochemical coloring method based on copper sulfate and dithiooxamide solutions. Digital images captured in an optical microscope were used to measure the percentage of copper ion penetration over the total enamel thickness on three sections taken from the exposed area. RESULTS Analysis of Deviance showed a significant difference for the permeability values among groups (P = 0.046). Likelihood ratio test revealed that the use of the CP10% and HP35% agents caused significantly greater enamel permeability than did the control treatment. No significant differences in percentage of copper ion penetration were found among CP10%, CP16%, CP37% and HP35%. This in vitro study demonstrated that depending on the external bleaching procedure, enamel permeability may be increased.