Background: Insecticides may have negative effects on reproductive organs. Given the interaction between leptin and the hypothalamic-pituitary-gonadal (HPG) axis, we sought to investigate the changes in leptin and the HPG axis in adult male rats poisoned with Proteus and Biscaya insecticides. Methods: Our experimental subjects were 110 adult male Wistar rats (80–90 days of age; average weight=200–210 g). They were randomly split into 11 groups of 10 rats: control, sham, and 9 experimental groups namely treatment with 2.75, 5.5, and 11 mg/kg/BW of Proteus, treatment with 1.5, 3, and 6 mg/kg/BW of Biscaya, treatment with 2.75 mg/kg/BW of Proteus+1.5 mg/kg/BW of Biscaya, treatment with 5.5 mg/kg/BW of Proteus+3 mg/kg/BW of Biscaya, and treatment with 11 mg/kg/BW of Proteus+6 mg/kg/BW of Biscaya. Intraperitoneal injections were performed over a 14-day period. For bloodletting at the end of the experiment, blood samples were withdrawn from the rats in order to investigate the serum concentration of luteinizing hormone (LH), follicle-stimulating hormone (FSH), gonadotropin- releasing hormone (GnRH), testosterone, and leptin. The data were analyzed using SPSS, version 16, via ANOVA and the Duncan test. A P value equal to or less than 0.05 was considered statistically significant.Results: Our comparisons between the experimental groups (average and maximum compound concentrations of Proteus and Biscaya) and the control group showed a significant decrease in the mean serum levels of FSH (P=0.001), LH (P=0.001), GnRH (P=0.001), testosterone (P=0.005), and leptin (P=0.001) in all the experimental groups in a dose-dependent manner.Conclusion: Proteus and Biscaya decreased GnRH, LH, FSH, and testosterone by reducing the serum level of leptin in the hypothalamus in a dose-dependent manner.