Fasting gastrointestinal motility in the human is characterized by the regular cycling activity of the migrating motor complex (MMC). Our purpose was to define the variability of the MMC within and between a group of six healthy subjects studied for 6-9 hr over six separate days with a perfused catheter system. A total of 88 phase III events was observed during 255 hr of recording in this group. The mean MMC cycling time varied significantly between subjects (range 113-230 min, P less than 0.001), and variation within subjects also was wide (SD range 58-70 min). Seventy-one percent of phase III events commenced in the gastric antrum, 18% in the proximal duodenum, 10% in the distal duodenum, and 1% in the proximal jejunum. For each subject, the velocity of propagation of phase III decreased significantly (P less than 0.001), and phase III duration increased significantly (P less than 0.001), with increasing distance from the os. In the antrum, phase I was predominant, and significant (P less than 0.006) variation between subjects was noted for percentage of MMC cycle occupied by phase I (overall mean +/- SD 55 +/- 23%). Phase II was predominant in both duodenum and jejunum (mean range 70-80%), and no significant variation was noted between subjects for percentage of MMC occupied by phase II. We conclude that human MMC activity varies widely between individuals and within the same individual when studied on separate days.