The effect of Ketanserin, a new antiserotoninergic drug, on human anal pressure in vivo was investigated. Anal pressure was recorded continuously in 14 normal subjects by a low-compliance water perfused probe with two recording points at the sphincter level. After a 30-min basal tracing Ketanserin (10 mg IV as bolus) or placebo was administered in a double blind manner, and the recording continued for 1 h. The results show that Ketanserin induced a 30% fall in anal pressure soon after its administration which was statistically significant when compared with the placebo (p less than 0.01). This effect lasted up to 40 min of recording and was followed by a return to control values within 1 h.