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The testes produce sperm in the seminiferous tubules and synthesize testosterone. Spermatogenesis depends on support from Sertoli cells, which form the blood-testis barrier and secrete inhibin and androgen-binding protein. Leydig cells produce testosterone in response to luteinizing hormone (LH). The hypothalamic-pituitary-gonadal axis connects gonadotropin-releasing hormone (GnRH), LH, and follicle-stimulating hormone (FSH) with testicular activity: LH stimulates Leydig cells, while FSH supports spermatogenesis through Sertoli cells. Testosterone and inhibin provide negative feedback. The topic also considers testosterone’s roles and how conversion by 5α-reductase produces DHT, which has distinct effects, including on external genitalia and prostate growth.