Protein phosphorylation/dephosphorylation is an important posttranslational modification that plays a critical role in signal transduction. The androgen receptor (AR) is under such control. We demonstrate that androgen receptor phosphorylation determines whether or not AR ligands perform as agonists or antagonists in LNCaP cells. Androgen receptor ligands (such as dihydrotestosterone and beta-estradiol) stimulate receptor expression and phosphorylation and, as a result, they act as agonists or partial agonists. In contrast, agents such as bicalutamide and estramustine inhibit the receptor phosphorylation and act as antagonists. This model is supported by gene expression and transactivation assays. Significant increases in levels of both mRNA and protein of prostate-specific antigen (PSA), a natural AR target gene, occur following the treatment of LNCaP cells with DHT, beta-estradiol, or hydroxyflutamide. In contrast, exposure of LNCaP cells to bicalutamide or estramustine results in a sharp decrease of PSA expression. Agonistic or antagonistic effect of these compounds on PSA expression parallels the level of phosphorylated, but not dephosphorylated androgen receptors. These agonistic or antagonistic effects are also observed in HeLa cells transfected with wild-type AR expression plasmid (pAR0) and AR-driven luciferase expression plasmid GRE-tk-LUC in the presence of different groups of AR blockers. Our data indicate that the functional status of androgen receptors is strongly correlated with the phosphorylation status of the receptors, and that the phosphorylated androgen receptor is the form of the receptor transcriptionally active in regulation. Thus the androgen receptor phosphorylation/dephosphorylation may serve as a new molecular target for screening androgen antagonists for the treatment of prostate cancer.