Background & Objectives : Nonylphenol (NP), a member of alkylphenols, has been widely used in manufacturing antioxidants, The present study was undertaken to examine whether short-term exposure to NP can alter the onset of puberty and associated reproductive parameters such as hormone receptor expressions in prepubertal female rats. Methods : NP (0.1, 1, 10 mg/kg BW) was administered orally at 10:00 from postnatal day 25 (PND 25) through the PND 35. The control group was administered orally each day 200 uL of the sesame oil. At PND 36, all groups were sacrificed at 18:00. The first vaginal opening (V.O.) day was monitored, and the weights of reproductive tissues were measured. Ovaries and uteri were used for analysis of mRNA levels, protein levels and histology. The transcriptional activities of the ovaries (StAR, P405scc, 3β-HSD, 17β-HSD, Aromatase and LH-R) and uteri(ER) were measured using quantitative RT-PCRs. The uterine proteins were used for the ER western blotting. Histological analysis of the reproductive organs was carried out. Results: Administration of NP induced early V.O. when compared to control. The ovarian weights of 1 mg group (0.164±0.010 mg/g BW) was significantly higher than those of control group (0.116±0.007 mg/g BW). Among the steroidogenesis-related genes, mRNA levels of the ovarian P450scc, 3β-HSD and Aromatase are increased significantly in all NP-treated groups. The mRNA levels of StAR and 17β- HSD were unchanged. NP administration (1 mg group) significantly increased the LH-R transcript level. Among the steroid hormone receptor genes, the mRNA level of ER-alpha was significantly elevated in 1mg NP group, while the mRNA levels of ER-beta and PR were unchanged in all NP-treated groups. Interestingly, wetern blotting data for uterine steroid hormone receptors revealed the complete disagreement with the transcriptional activities. Histological status of ovaries and uteri also exerted the occureence of advanced maturation in NP-treated groups. Conclusion: The present study demonstartes that the short-term administration of NP accelerates V.O. and maturation of reproductive organs such as ovary and uterus.