A role for steroid hormones has been proposed for the post-pubertal factor IX increment of approximately 25% seen in both normal males and females, as well as in the post-pubertal phenotypic recovery seen in haemophilia B Leyden. We have evaluated androgen receptor binding to the factor IX promoter and have assessed transcriptional activation of the factor IX gene in hepatocytes through transient transfection studies and through expression of factor IX in a murine model of androgen insensitivity. Whereas transfection of the androgen receptor alone did not activate expression from the factor IX promoter, co-transfection with the CCAAT enhancer binding protein resulted in a synergistic 17-fold enhancement of transcriptional activity. Using liver nuclear extracts and recombinant androgen receptor protein we have confirmed binding of this protein to the factor IX proximal promoter and disruption of binding with a mutation at nucleotide -26. Finally, studies in normal and testicular feminized male mice showed different developmental patterns of factor IX expression. In normal mice, expression recapitulates that seen in humans, with early post-natal levels being approximately 50% of the adult values and with a post-pubertal increment of approximately 25%. In contrast, testicular feminized animals did not show a significant post-pubertal increment of factor IX. These studies provide further support for the role of androgen receptor binding to the factor IX promoter in regulating the developmental expression of factor IX.