Objective(s):Methamphetamine (METH) is a psychostimulant that has harmful effects on all organs, the nervous system, cardiovascular system, and reproductive system. Since many METH consumers are young people of reproductive age, it poses a risk to the next generation of METH consumers. METH can pass through the placenta and is also secreted into breast milk. Melatonin (MLT) is the primary hormone of the pineal gland that regulates the circadian cycle, and it is also an antioxidant that can mitigate the effects of toxic substances. This study aims to investigate the protective effect of melatonin against the detrimental effects that METH has on the reproductive system of male newborns, whose mothers consumed METH during pregnancy and lactation.Materials and Methods:In the current study, 30 female adult balb/c mice were divided into three groups: control group, vehicle group that received normal saline, and the experimental group that received 5 mg/kg METH intraperitoneally during gestation and lactation. After lactation, the male offspring of each group were randomly divided into two subgroups, one of which received 10 mg/kg melatonin intragastrically for 21 days (corresponding to the lactation period of the mice) (METH-MLT) and the other did not (METH -D.W). After treatment, the mice were sacrificed and testicular tissue and epididymis were obtained for the following tests.Results:The diameter of seminiferous tubules, SOD activity, total Thiol groups concentration, catalase activity, sperm count, and PCNA and CCND gene expression were significantly increased in the METH-MLT group compared with the METH-DW. Apoptotic cells and MDA level ameliorated in the METH-MLT group compared with METH-D.W, and testicular weight had no notable change.Conclusion:The current study represents that consumption of METH during pregnancy and lactation can have adverse effects on the histological and biochemical factors of testis and sperm parameters of male newborns, which can be mitigated by taking melatonin after the end of the breastfeeding period.