Background: Recent breakthrough therapies targeting androgen receptor signalling in CRPC involve multifunctional androgen receptor (AR) blockade and exhaustive androgen deprivation. Nevertheless, limitations to an enduring effectiveness of new drugs are anticipated in resistance mechanisms occurring under such treatments. Here we specify the origin of CYP17A1 up-regulation under abiraterone acetate treatment in CRPC cell models. Methods: CRPC cell models VCaP and LNCaP as well as AR-negative PC-3- and non-neoplastic epithelial BPH-1-cells were treated with various concentrations of abiraterone hydrolyzed from abiraterone acetate (AA, Janssen-Cilag, Germany). Functional analyses by RNA interference were done with AR- and CYP17A1-siRNA. Expression of androgen regulated genes and CYP17A1, AKR1C3 was quantitated by real time PCR. Protein analyses were performed by western blots and PSA secretion was assessed by ELISA kits. Results: AA treatments (5 to 25 µM) of AR positive CRPC cell models led to decreased expression of androgen regulated genes such as PSA. In these cells diminished expression of androgen regulated genes was accompanied by an up-regulation of CYP17A1 expression (>2-fold at 25µM AA) within short treatments of 24 hours. No such effects became evident in AR-negative PC-3 cells. AR directed siRNA (siAR) used in VCaP cells significantly reduced mRNA expression and AR protein abundance. Such interference with AR signalling in the absence of AA also caused an (up to 8-fold) up-regulation of CYP17A1 expression. Conclusions: In AR-positive cells AA treatments cause a down-regulation of androgen regulated genes in spite of an elevated expression of CYP17A1, the very target enzyme for this drug. CYP17A1 up-regulation already occurs within 24 hour treatments of AA and does not require adaptation events over several cell cycles. CYP17A1 is also up-regulated in the absence of AA when AR signalling is physically eliminated by siAR. These results reveal an immediate counter-regulation of CYP17A1 expression whenever AR-signalling is inhibited adequately but not a persisting adaptation yielding drug resistance.