This study investigates whether measurements of anal sphincter pressure and length are reproducible and whether visual analyses of manometric tracings are reliable. Manometric studies were performed on three different days in 10 male and 10 female healthy subjects with the use of a pneumohydraulic capillary perfusion system. There was complete agreement between both observers in the analysis of anal resting and squeeze pressure as well as sphincter length. The pressure profiles from different days correlated significantly (P less than 0.01) with each other regardless of whether the studies were performed in the prepared or unprepared bowel. However, anal sphincter length and the quality of the anorectal inhibitory reflex varied greatly on different days. Male and female subjects showed major differences in the capacity of the anal sphincter to contract voluntarily (P less than 0.01), but resting tone and sphincter length were comparable in the two groups. It is concluded that pressure determinations in the anal canal are reliable and reproducible, but that alterations in the quality of the anorectal reflex need to be confirmed by repeated measurements before pathology is diagnosed.