The aim of this study was to explore the mechanism underlying the dual effect of androgen on prostate cancer cells and further explore its correlation with dopa decarboxylase (DDC), an androgen receptor (AR) coactivator and a traditional neuroendocrine differentiation (NED) marker. Cell proliferation and cycling after treatment with synthetic nonmetabolizable androgen R1881 was determined by the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide) method and flow cytometry. Differential gene expression was analyzed by cDNA microarrays. DDC expression during the dual effect of R1881 was further explored with microarray, quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), Western blot, and enzyme activity assays. Proliferation of LNCaP cells was inhibited by 1 nM R1881 but stimulated by 0.1 nM R1881. Compared with the untreated cells, 320 (2.26%; 170 up-regulated, 150 down-regulated) and 4608 (32.65%; 2046 up-regulated, 2562 down-regulated) genes were found to be expressed differentially in the 1 nM and 0.1 nM R1881-treated cells, respectively. The results were partially confirmed by RT-PCR and Western blot. The DDC gene was down-regulated in the 1 nM R1881-treated cells and up-regulated in 0.1 nM R1881- and 30 nM hydroxyflutamide-treated cells. The enzymatic activity of DDC in the latter 2 groups was also strengthened. Meanwhile, the NED markers CgA and synaptophysin were not affected by these AR activators. R1881 had a dose-dependent biphasic effect on LNCaP cell proliferation. AR coactivator DDC was respectively down- and up-regulated in high and low concentrations of R1881. DDC up-regulation by exogenous AR activators is not accompanied by up-regulation of definitive NED markers.