Final answer:
Luteinizing hormone (LH) stimulates Leydig cells to produce testosterone. This process is initiated by GnRH from the hypothalamus, which induces the pituitary to release LH and FSH.
Step-by-step explanation:
The hormone responsible for stimulating Leydig cells in the testes to produce testosterone is the luteinizing hormone (LH). The process begins with the hypothalamus releasing gonadotropin-releasing hormone (GnRH), which signals the pituitary gland to release LH and follicle-stimulating hormone (FSH). While FSH primarily acts on Sertoli cells to aid in spermatogenesis, it is LH that specifically binds to the Leydig cells, triggering them to synthesize and secrete testosterone, crucial for male reproductive functions.