The effects of postnatal exposure to octylphenol(OP) on the expressions of the steroidogenic enzymes and testosterone production were evaluated. Postnatal male mice (15-day-old) were injected with 2 or 20mg $kg^{-l}$ body weight (BW) of OP for 5 days and sacrificed on postnatal day 21. Testosterone concentration was measured by radioimmunoassay and the expressions of the testicular genes were determined by RT-PCR analyses. Significant reductions in the mean body and testis weight were observed in the OP treated animals. No marked alteration in the histological structure of the testis were observed, however, slight reduction in the seminiferous tubule diameter and the number of Leydig cells and several pyknotic cells could be identified in the 20 mg $kg^{-l}$ BW of the OP treated animals. Serum testosterone concentration was dramatically reduced and the mRNA expressions of the steroidogenic acute regulatory protein (StAR), cholesterol side-chain cleavage enzyme (P450scc) and $17eta$-hydroxylase/Cl7-20 lyase $(P450_{17alpha})$ were decreased. No significant changes of the gene expressions of the steroidogenic factor-l (SF-I) and estrogen and androgen receptor after the OP treatment showed that the decreased expressions of the steroidogenic enzymes in the present study did not correlate with these genes. Altogether, the present study demonstrates that postnatal treatment of OP inhibits steroidogenesis by decreasing the transcriptional expressions of the StAR and steroidogenic enzymes. The alteration in steroidogenesis may adversely affect the normal development of the testis and sper- matogenesis.