Nitric oxide is an important mediator of gut smooth muscle relaxation and visceral sensation. Sildenafil results in stimulation of the nitric oxide-cyclic GMP pathway. We sought to determine the effects of daily sildenafil administration on colorectal function. Over a 4-week period, sildenafil was administered during weeks 2 and 3. Stool frequency and consistency were assessed daily. Anorectal manometry, rectal sensation, and colon transit testing were performed at the end of weeks 1 and 3. Ten healthy men were studied. No significant differences in segmental or total colon transit time were noted; however, significant changes in stool frequency and trends toward decreased stool consistency were noted during sildenafil use. A trend toward reduced resting anal sphincter pressure was seen after sildenafil. Rectal volumes to first sensation and desire to defecate were significantly increased after sildenafil on test day 2 only. Additionally, volumes to desire to defecate and maximal tolerable volume were significantly increased before sildenafil on test day 2 compared to before sildenafil on test day 1. We conclude that daily administration of sildenafil is well tolerated and results in alterations in colorectal function.