Rectal sensitivity is often reduced in patients affected by chronic constipation, but it is not known whether this alteration differs according to the severity and the site(s) of the slowing of gastrointestinal transit. Moreover, it is not known whether alteration precedes or follows bowel complaints. In this study, perception of intrarectal distension was evaluated in 28 healthy controls, in 20 patients complaining of constipation and with a normal gastrointestinal transit time (less than 96 hr), and in 44 patients complaining of constipation and with a prolonged gastrointestinal transit time (greater than 96 hr). Within the latter group, perception to intrarectal distension was analyzed in patients with slowing of transit in the rectum only, in the colon only, and in both the rectum and the colon. In a subgroup of 22 patients, rectal sensitivity was evaluated before and after treatment. Rectal sensitivity was found to be reduced significantly in constipated patients; it was more severely reduced in patients with objective evidence of prolonged gastrointestinal transit time and with slow transit in the rectum. Rectal sensitivity improved in patients who responded to treatment and did not vary significantly in nonresponders.