Background: Sildenafil citrate$(Viagra^{circledR})$ improves sexual functions in men with erectile dysfunction and is widely prescribed. But little is known about the effects of sildenafil citrate on autonomic nervous system. Methods : Seventeen health male subjects$(age;26{sim}36;years)$ were investigated. Blood pressure (BP), heart rate(HR), and autonomic nervous function tests, such as BP rise in the sustained hand-grip, sympathetic skin reflex(SSR), HR variation with deep breathing(HR variation), and Valsalva ratio, were measured before and 60 minutes after administration of a single 50mg oral dose of sildenafil citrate. Results : Systolic BP were slightly decreased after sildenafil citrate administration $(118.8{pm}11.77mmHg{ ightarrow}112.7{pm}13.8mmHg,;p=0.048)$. HR and diastolic BP were not changed significantly after sildenafil citrate administration. And autonomic nervous function tests were not changed significantly after sildenafil citrate. Conclusions : Sildenafil citrate does not cause significant changes in autonomic nervous system except systolic BP.