Pyrethroids are insecticides with worldwide commercial and retail usage and they provide benefits such as increased supply of crops, fruits and vegetables, as well as control of diseases transmitted through insects that affect humans and livestock. Despite their benefits, many pesticides are neurotoxic compound that induce acutely toxic effect at high doses and can potentially exert more subtle effects at lower doses though different exposure routes. The permethrin is one of the most widely used pyrethroids that in mammals, once absorbed, is rapidly distributed to various organs/tissues and is transformed into metabolites, 3-(2,2-dichlorovinyl)-2,2-dimethyl- (1-cyclopropane) carboxylic acid (DCCA) and the 3-phenoxybenzoic acid (3-PBA). The 3- PBA has been used in numerous studies for the biomonitoring of exposure in occupationally and environmentally exposed individual. The aim of our work was to develop an analytical method for the quantification of 3-PBA in hair samples. Hair is a non-invasive biological material useful in the biomonitoring of trace elements because it is a vehicle for substance excretion from the body, and it permits evaluating longterm exposure. Our animal’s model are rat exposed to permethrin in early days of them life and we took hair rat’s samples after 22 and 60 days. The hair has been analyzed with the purpose to assess if 3-PBA content could be used as biomarkers on this sample. The analytical method comprises five main steps: preparation of hair samples, decontamination step, extraction of analytes, derivatization and subsequent instrumental analysis by gas chromatography (GC) and detection using ion trap mass spectrometry (MS). The analytes were isolated from hair with recoveries of 78%. The limit of detection and quantification ensured by the method are 0.92 µg/mg and 3.06 µg/mg, respectively. The results presented in this work show the potential of GC-MS based method for the extraction of 3-PBA in hair of rat aged 22 days, but I could not be determined the 3-PBA concentration in hair of rat aged 60 days.