Anal and rectal pressures and external sphincter electromyogram were recorded continuously during rectal infusion of 1.5 L saline in 18 normal subjects and 37 patients who complained of diarrhea and fecal incontinence. All subjects exhibited a pattern of regular fluctuations in anorectal pressure and electromyogram. All except 1 of the normal subjects were able to retain 1500 ml saline without leakage, and their pressure record comprised simultaneous rectal contractions, internal sphincter relaxations, and external sphincter contractions. None of the incontinent patients were able to retain 1500 ml saline without leakages, and leakages always coincided with the peaks of rectal pressure. Two manometric patterns were observed. Fifty-nine percent of incontinent patients exhibited a pattern of contractions of similar profile occurring throughout the anorectum. This finding was associated with low basal sphincter pressures, an easily inhibited anal sphincter tone, an obtuse anorectal angle, and a funnel-shaped configuration to the anal canal. These results suggested that, in this group, the internal sphincter was weak and easily inhibited so that the whole anorectum behaved as one fluid-filled compartment recording contractions of the external sphincter. The remaining 41% of incontinent patients exhibited a normal pattern of anorectal pressure fluctuations and had normal maximum basal pressures, although maximum squeeze pressures, rectoanal inhibitory reflex, and anorectal angles were abnormal. Peak rectal pressures were abnormally high in this group during saline infusion, suggesting that abnormally strong rectal contractions may play a role in the incontinence in this group.