Transgenic female mice overexpressing the a- and b- subunits of human chorionic gonadotropin (hCGab+) exhibited precocious puberty, as evidenced by early vaginal opening. Chronically elevated hCG in 21-day-old hCGab+ females stimulated gonadal androgen production, which exerted negative feedback over the endogenous gonadotropin synthesis, and activated the hypothalamic GnRH pulsatility and gene expression. Transgenic females also exhibited elevated hypothalamic aromatization in the preoptic area (POA), which is the sexually-differentiated area that controls the LH surge in adulthood. Ovariectomy at 14 days of age was unable to rescue this phenotype. However, the blockade of androgen action by flutamide from postnatal day 6 onwards reduced the aromatase levels in the POA of hCGab+ females. Our results suggest that early exposure of females to androgen action during a critical period between postnatal days 6–14 induces sex-specific organizational changes of the brain, which affect the aromatase expression in the POA at the onset of precocious puberty.