['Prostate cells are dependent on androgens for growth and proliferation. Androgen deprivation therapy is the recommended treatment for advanced/metastatic prostate cancer. Under this therapy, prostate cancer will inevitably progress to castration resistant prostate cancer (CRPC). Despite putative castration resistance, testosterone might still play a crucial role in the progression of CRPC. The goal of this study was to determine the role of testosterone in the formation of metastases of CRPC in both in vitro and in vivo settings.', 'In vitro, the effect of testosterone and the non-aromatizable androgen methyltrienolone on migration, invasion and proliferation of a castration-resistant prostate cancer rat cell line (Dunning R3327-MATLyLu) was assessed using a transwell assay and a sulforhodamine B assay and immunohistochemical detection of ki67. Androgen receptor status was determined using Western blot. In vivo, Copenhagen rats were divided in four groups (males, females, castrated males and females with testosterone suppletion) and inoculated with MATLyLu cells. Tumor size was assessed daily.', 'Testosterone increased cell migration and invasion in a concentration-dependent manner in vitro. Testosterone did not affect in vitro cell proliferation. No difference was shown between the effect of testosterone and methyltrienolone. In vivo, in groups with higher levels of circulating testosterone, more rats had (micro)metastases compared with groups with low levels of testosterone. No effect was observed on primary tumor size/growth.'] Does serum testosterone play an important role in the metastatic ability of castration resistant prostate cancer?