Follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone concentrations were measured in serum of adult male rats after 6 days of constant subcutaneous infusion of varying levels of dihydrotestosterone (DHT). Doses from one-half up to the normal "blood production rate" of DHT produced a selective stimulation of serum FSH, but not LH, levels. Higher levels suppressed FSH, LH, and testosterone. Despite the presence of much higher levels of testosterone in blood, the augmentation of FSH secretion indicated in these studies suggests that DHT may have an important role in regulatory systems for gonadotropins.