The brain mechanism regulating GnRH/luteinizing hormone (LH) release is sexually differentiated in rodents. Estrogen induces a GnRH/LH surge in females but not in males. Kisspeptin neurons in the anteroventral periventricular nucleus (AVPV) have been reported to be sexually dimorphic and suggested to be involved in the GnRH/LH surge generation. Neonatal testicular androgen may cause the reduction of AVPV kisspeptin expression and a lack of LH surge in male rats. Thus, it is plausible that perinatal testicular androgen causes defeminization of the AVPV kisspeptin system, resulting in the loss of the surge system in male rats.