In the present study we determined if NELL2, a neural tissue-specific protein containing 6 epidermal growth factor (EGF)-like repeat domains, plays an important role in the regulation of puberty initiation in the rat hypothalamus. We origin811y found that NELL2 is a new estrogen-responsive gene in hypothalami derived from estrogen-sterilized and control rats using a PCR differential display. In the 40-day-old female rat hypothalamus, NELL2 was up-regulated by neonatal estrogen treatment. In situ hybridization histochemistry showed that NELL2 is very abundant in the ventromedial hypothalamic nucleus that is responsible for the control of sex behavior. NELL2 mRNA level in the medial basal hypothalamus showed a dramatic increase before female puberty onset, which suggests that NELL2 may be involved in the process regulating female puberty onset. We attemped to block NELL2 synthesis with intracerebroventricular injection of an antisense oligodeoxynucleotide (ODN) to the NELL2 mRNA, and examined its effect on the puberty onset of the female rat. The antisense ODN significantly delayed puberty initiation determined by vaginal opening. In summary, NELL2 may play an important role in the regulation of female puberty onset.