Because sildenafil (Viagra) is scheduled to be clinically available in the near future, urologists in Japan must prepare for changes in the treatment of erectile dysfunction. The drug will surely influence our current clinical style of diagnosis and treatment of the dysfunction. We herein discuss this issue, referring to favorable and unfavorable effects that will be brought about by the drug. We also propose several future studies that we should do, including those on testosterone replacement therapy. We present results of our experiment using testosterone replacement therapy in aged rats and speculate about the mechanism by which the hormone replacement works in the sex center of the brain.