Abstract: Background: Nickel alloys (Ni) are broadly used in the fabrication of orthodontic adjuncts, they become an integral part of almost every orthodontic interventions. It is suggested that nickel is a toxic metal and in some cases is capable of producing carcinogenic effects, therefore, it is important to measure its levels in individuals with orthodontic appliances. Objective: To determine the levels of nickel in the oral cavity through samples of saliva, biofilm and the oral mucosa before and during 6 months of the orthodontic treatment. Methodology: Quasi-experimental study, the size of the sample was stablished according to historical trend with 270 samples taken from 30 subjects to which orthodontic appliances were placed. In order to determine the nickel concentration, the samples were analyzed in a thermo scientific Atomic Absorption Spectrophotometer (AAS) iCE 3000 series with graphite furnace in three different moments and time use of the orthodontic appliances in the mouth. Results: When comparing the nickel concentration in 3 times and taking into account the type of sample, significant differences were found in the biofilm, no differences were found according to the commercial house and the type of sample that had the greatest nickel concentration in the 3 times was the biofilm. The nickel levels in the oral cavity change after the placement of the orthodontic appliances, those changes being more significant in the saliva and biofilm samples, the biofilm sample is the one with the greater nickel concentrations before and after the placement of the orthodontic appliances. Conclusion: The nickel levels showed a significant increase in the subjects of the sample. Especially in biofilm sample in relation to time of installation of the brackets appliances in the mouth.