Introduction. External beam radiotherapy for prostate cancer leads to erectile dysfunction in 36%-43% of patients. The underlying mechanism is largely unknown, although some clinical studies suggest that the arterial supply to the corpora cavernosa is responsible. Two animal experimental studies reported on the effects of a single fraction of prostate irradiation on the penile structures. However, irradiation in multiple fractions is more representative of the actual clinical treatment. Aim. The present prospective, controlled study was initiated to investigate the effect of fractionated prostate irradiation on the arteries of the corpora cavernosa. Main Outcome Measures. Histological evaluation of the penile tissue in comparison with control rats at 2, 4, and 9 weeks after irradiation. Methods. The prostate of twelve rats was treated with external beam radiation in 5 daily fractions of 7.4 gray. Three control rats were treated with sham irradiation. Prostatic and penile tissue was evaluated for general histology (hematoxylin-eosin). The penile tissue was further evaluated after combined staining for collagen (resorcin fuchsin) and alpha-smooth muscle actin (SMA) (Biogenex). Results. The prostate showed adequate irradiation with fibrosis occurring at 9 weeks after irradiation. The corpora cavernosa showed arteries that had developed loss of smooth muscle cells expressing SMA, thickening of the intima, and occlusions. All the control rats maintained normal anatomy. Conclusion. This is the first animal experimental study that demonstrates changes in the arteries of the corpora cavernosa after fractionated irradiation to the prostatic area. The preliminary data suggests that erectile dysfunction after radiotherapy might be caused by radiation damage to the arterial supply of the corpora cavernosa. van der Wielen GJ, Vermeij M, de Jong BWD, Schuit M, Marijnissen J, Kok DJ, van Weerden WM, and Incrocci L. Changes in the penile arteries of the rat after fractionated irradiation of the prostate: A pilot study. J Sex Med 2009;6:1908-1913.