Background: Sildenafil citrate is effective in men with erectile dysfunction. Sildenafil induces degenerative changes in seminiferous tubules and interstitial histological alterations. Diabetes mellitus may cause an end-organ damage in many systems as the genitourinary system. The testis is the primary sex organ and is formed mainly of stroma and parenchyma. It is a mixed gland that performs both endocrine and exocrine activities. Aim of the study: This work aimed to reveal the effect of sildenafil citrate in high and toxic doses on the histological structure of the healthy and diabetic rat testes . Materials and Methods: The research was carried out at the animal house at Faculty of Pharmacy, Al-Azhar University between July, and September 2021. Fifty adult rats were used. They were divided into the following groups: Group I: 10 rats received distilled water daily; group IIA: 10 rats received intraperitoneal injection of sildenafil 9mg/kg/d; group IIB: 10 rats received sildenafil 13.5mg/kg/d, group IIIA: received a single dose of an intraperitoneal injection of 150 mg/kg body weight of alloxan to induce diabetes then sildenafil citrate 9 mg/kg/d, group IIIB: received a single dose of an intraperitoneal injection of 150mg/kg of alloxan to induce diabetes then intraperitoneal injection of sildenafil citrate 13.5mg/kg/d. Animals from all groups were examined 2 months after the start of the experiment. Results: Sildenafil had a toxic effect on the testes of adult albino rats that was more severe with the toxic dose. Also, the changes were mild in non-diabetic and moderate to severe in diabetic rats. Conclusion: Sildenafil had toxic effects on the testes in high doses and also in diabetics.