PURPOSE: Androgen plays an important role during penile development and is essential for a normal libido in the male, but its role in the regulation of the androgen receptor (AR) and maintenance of the erectile response has been controversial. We investigated the effect of androgen on the AR and apoptosis of the penile erectile tissue after castration. MATERIALS AND METHODS: Adult Sprague-Dawley rats were divided into four groups: sham operation, castration, testosterone and dihydrotestosterone (DHT) replacement after castration. Androgens (testosterone, DHT) were administrated for 7 days at week 1,2,3, and 4 after castration. The AR mRNA expression detected by reverse transcription-PCR and percentage of apoptosis (apoptotic index) were analyzed in the penis. The relation between androgen and p53 was determined by Western blot analysis. RESULTS: Castration induced a progressive decrease in serum testosterone. As serum testosterone decreased, a reduction in AR mRNA expression was noted, along with an increase in apoptosis and p53 expression in the penis. Androgen replacement after castration increased the AR mRNA expression with a decrease in apoptosis and p53. CONCLUSIONS: The adult rat penis was affected by the androgen milieu via AR expression and apoptosis. Therefore, androgens such as testosterone and DHT plays a direct role in the erectile function in adult rat.