BACKGROUNDMercuric chloride (HgCl2) is poisonous to humans and animals and typically damages the nervous system and other organs. Mercuric chloride exposition disclosed to initiation of oxidative stress pathway can result in a defect in male fertility and testis tissue. Synthesized selenium nanoparticles (SeNPs) were characterized with a diameter range minimal than 100 nm, having the effective sets of the biological matter. The present study aimed to evaluate the effect of biosynthesized SeNPs, prepared by leek extract on Wistar rats' testicles and brain.METHODSThirty‐five Wistar male rats (120–150 g) were randomly split into five groups (n = 7), orally ingested with leek aqueous extract loaded on SeNPs, and then the animals were administered with mercury II chloride (HgCl2) to induce testis injury and damage the nervous system.RESULTSThe used dose of mercuric chloride led to oxidative stress damage in the testis of the rats which was evidenced by a decrease in testosterone, luteinizing hormone (LH), follicle‐stimulating hormone (FSH) and proliferating cell nuclear antigen (PCNA) levels, and an increase in nuclear factor‐kappa B (NF‐κB) and caspase‐3. Also, HgCl2 decreased the levels of dopamine (DA), serotonin (5‐HT), norepinephrine (NE) and brain‐derived neurotrophic factor (BDNF) in the brains of rats. In addition, A decrease was observed in the levels of antioxidant markers, B‐cell lymphoma‐2 (Bcl‐2), as well as an increase in malondialdehyde (MDA), nitric oxide (NO), NF‐κB, tumor necrosis factor (TNF)‐α, interleukin (IL)‐1β and Bax in both testes and brains. Pre‐treatment with leek extract loaded on SeNPs significantly ameliorated testosterone, LH, FSH, PCNA and caspase‐3 levels in the testis and DA, 5‐HT, NE and BDNF in brains. Although the contents of MDA, NO, TNF‐α, IL‐1β, NF‐κB and Bax decreased significantly in both. glutathione, glutathione peroxidase, glutathione reductase, catalase, superoxide dismutase and Bcl‐2 levels were significantly improved in both organs.CONCLUSIONOur findings suggest that treatment with aqueous leek extract loaded on SeNPs may offer promising prospects for the advancement of anti‐inflammation activity against testis injury and also have a very key role in neurobehavioral alterations as a result of mercury toxicity. © 2024 Society of Chemical Industry.