Patients with IBS have visceral hypersensitivity. Serotonergic modulation can alter visceral perception and be of benefit in some patients with IBS; however, it is unclear whether serotonergic modulation alters rectal motor function in these patients. In a randomized, double-blind, placebo-controlled, crossover study, acute tryptophan depletion and citalopram (a selective serotonin reuptake inhibitor) did not affect rectal motor function in healthy control individuals or patients with diaorrhea-predominant IBS.