Bakground & Objective: Since Olanzapine is used in the treatment of psychosis and schizophrenia, the aim of this investigation was to assess the effect of Olanzapine on the pituitary –gonad axis and spermatogenesis in adult male rats. Materials & Methods: In the present research, 50 male Wistar rats were divided into 5 groups of 10, including control, sham, and experimental groups I, II, and III, which orally received 5, 10, and 20 mg/kg Olanzapine solution for 28 days, respectively. Twenty-four hours after the last treatment, all the rats were weighed and anesthetized. Blood samples were obtained from the heart and were centrifuged; the sera were thereafter evaluated for concentrations of LH, FSH, testosterone, and prolactin via the RIA method. Results: The results show that there was a significant increase in the body weight of the group receiving 20 mg/kg Olanzapine solution. In addition, the results of hormone assay demonstrated that the serum concentrations of LH and FSH were comparable between the experimental groups, while there was a significant increase in prolactin and significant decrease in testosterone in the group receiving 20 mg/kg Olanzapine solution. Conclusion: The changes observed in the subjects were due to the presence of compounds in the Olanzapine solution, especially Benzodiazepine, which affected the body weight and serum concentration of prolactin and testosterone in the groups receiving 20 mg/kg Olanzapine solution