ABSTRACT.Qualitative changes are known to occur in testicular steroidogenesis at birth as the testosterone peak is reached without significant elevation of basal luteinizing hormone in the 2nd wk of life. This study was designed to evaluate testicular activity prior to these changes. Pituitary-testicular function was studied by measuring serum gonadotropins and steroids after stimulation by gonadotropin-releasing hormone (GnRH) (one intravenous injection) and human chorionic gonadotropin (hCG) (three intramuscular injections). The subjects had minor genital anomalies; their ages ranged from 2 to 6 days. All had a strong luteinizing hormone response and a weaker follicle-stimulating hormone response to GnRH stimulation. hCG induced significant increases in serum pregnenolone, 17-hydroxyprogesterone, androstenedione, testosterone, and dihydrotestosterone. Serum estradiol and estrone did not change, and progesterone decreased. The results clearly show that the pituitary-testicular axis is functional neonatally. The responsiveness of the testis to hCG supports the assumption that the postnatal decrease of testicular steroids is due to the simultaneous disappearance of hCG from the circulation. The neonatal testis does not show any estradiol response to hCG, which is a feature typical of prepuberty.