As a key steroidogenic enzyme,cytochrome P450c17(CYP17,17α-hydroxylase/17,20-lyase)plays a critical role in the production of sex steroid hormone in many teleost.In order to evaluate the functions of P450c17-I during the gonad development and the relationship between its expression and sex steroid hormone levels,we first cloned and characterized the full-length cDNA of P450c17-I in Navodon septentrionalis.The full-length cDNA of CYP17-I is 1 580 bp,encoding 511 amino acids.The analysis of the structure shows that CYP17-I contains relatively conserved sequence.Then RT-PCR method was used to check the expression of the CYP17-I in different tissues.The result shows that the higher expressions are in ovary,testis and brain,there is relatively low expression in heart,spleen,kidney,and there is much lower expression in pituitary and gill.In addition,the method of ¹²⁵I labeled radioimmunoassay(RIA)is applied to examine the changes of serum testosterone(T),17β-Estradiol(E₂)levels during the reproductive cycle.The results show that the female fish has the highest level of E₂ at the early stage of vitellogenesis and it decreases along with spawning.In the male fish the level of E₂ was not significantly different in the reproductive cycle.But the level of T in both male and female fish was developmental stage-dependent and the highest level of T appear at 5th month.RT-PCR showed that the expression patterns of CYP17-I were developmental stage-dependent,which is consistent with the changes of T in male fish and female fish.And the highest expressions of CYP17-I are found in the gonad of both genders before the pre-ovulation phase in the reproductive season.But there is no obvious relationship between E₂ and CYP17-I expressions.These suggest that CYP17-I plays a critical role in the production of male testosterone hormone in N.septentrionalis.