To investigate anal sphincter mechanics, anal pressure was measured in 14 normal males and 11 normal females using probes of 0.4 to 3 cm in diameter. Resting pressure profiles on insertion and withdrawal did not differ significantly. Anteroposterior pressure differences could be explained by leverage of rigid probes against the anterior rectal wall. A maximal voluntary squeeze increased pressure throughout the anus, whereas the recto-anal inhibitory reflex resulted in a greater reduction in pressure in the upper part of the anal canal. Resting pressure, squeeze pressure and minimum residual pressure (during rectal distension) rose with increasing anal diameter. Estimated sphincter tension was linearly related to anal diameter and the slope of this relationship was increased by sphincter contraction and reduced by sphincter relaxation. The deviation from linearity of this relationship at low anal diameters may be due to swelling of the anal cushions to maintain anal pressure when muscular tension approaches zero.