['To investigate the effects of testosterone on cavernosal endothelial progenitor cells (EPCs) in a castrated rat model.', 'In all, 45 male Sprague-Dawley rats (12-weeks old) were divided into control, surgical castration, and castration with testosterone replacement groups. The rats were castrated under ketamine anaesthesia, and testosterone was administered by daily subcutaneous injection of 3 mg/kg testosterone propionate. The corpus cavernosum was obtained after perfusion with 10 mL saline via the abdominal aorta 4 weeks later. The expression of EPC-specific markers [cluster of differentiation 34 (CD34), fetal liver kinase 1 (Flk1), and vascular endothelial (VE)-cadherin] was evaluated by flow cytometry analysis and immunofluorescence staining.', 'CD34+/Flk1+ and CD34+/VE-cadherin+ cells were detected in the cavernosal sinusoidal endothelial space. Flow cytometry analysis showed that CD34 and Flk1 double positive cells (EPCs) comprised ≈3.79% of the corpus cavernosum in normal rats. The percentage of EPC marker-positive cells decreased significantly in the castration group (2.8%; P < 0.05) and was restored to 3.56% after testosterone supplementation. Confocal microscopy revealed that the numbers of CD34+/Flk1+ and CD34+/VE-cadherin+ cells decreased in castrated rats compared with controls, but were similar to control levels in rats receiving testosterone replacement.'] Does testosterone modulate endothelial progenitor cells in rat corpus cavernosum?