Manometric studies of the sigmoid colon were performed on 17 healthy volunteers and on 49 constipated patients, after a long period of fasting (18--20 hours). Motility was recorded using perfused catheters at basal level during 45 minutes, then 60 minutes after a 0.5 mg intravenous injection of neostigmine, and, finally, 30 minutes during and after a meal. Motor activity was assessed by a motility index (per cent of activity x mean amplitude of waves). In both normal and constipated patients, the basal motility index was very low (respectively 82 +/- 16 and 110 +/- 113). This low level of activity was due to the long fasting period imposed on all the subjects. After neostigmine the motility index increased in both controls (347 +/- 256) and constipated patients (311 +/- 325); this test, however was found to be unreliable. The meal increased the motility index to significantly higher values than after neostigmine in controls (538 +/- 215). In constipated patients the mean meal motility index was comparable with that of controls (577 +/- 549) with a large distribution of individual values. Using the mean meal motility index +/- 2 SD of the control group as a term of comparison, the patients were segregated into three groups: 'hypomotor' patients (eight cases), 'normomotor' patients (33 cases), and 'hypermotor' patients (eight cases). From the evidence of this series of clinically well-defined constipated patients, it was concluded that only the meal test is able to segregate three significant patterns of sigmoid activity and that a large number (68%) of constipated patients exhibit normal sigmoid motor activity.