It has been reported that penile PDE5 expression was under androgen regulation. However it remained unknown whether the observed change in PDE5 expression in castrated animals was under direct androgen regulation or due to changes in smooth muscle content. In the present study we showed that castration of rats caused a reduction of penile size and cavernous smooth muscle content. Immunostaining detected concomitant reduction of PDE5 and alpha smooth Muscle actin (alpha-SMA) expression in the corpus cavernosum of castrated rats. Real-time PCR and Western blotting detected no change of PDE5 expression when normalized with alpha-SMA expression in castrated rats. Androgen receptor (AR) expression was increased while PDE5 expression remained unchanged in DHT-treated rat cavernous smooth muscle cells (CSMC). Prostate specific antigen (PSA) promoter activity Was upregulated while PDE5A promoter activity remained unchanged in DHT-treated CSMC. Thus, PDE5 expression was not under direct androgen regulation. (C) 2009 Elsevier Inc. All rights reserved.