To elucidate the specific function of the three phases (I-III) of the migrating motor complex (MMC) by manometry, detailed analysis of individual pressure waves in the proximal duodenum was performed. Twenty healthy subjects (10 men and 10 women of whom 11 were tube-naive) underwent computerized manometry for 5 h during fasting followed by 45 min after a meal using an 8-channel water perfused catheter. Three recording points were in the antrum, three in the proximal duodenum (2 cm apart), one in the distal duodenum and one in the proximal jejunum. In all subjects at least one phase III (median 2) was observed during the 5-h fasting recording. In the proximal duodenum the mean proportion of retrograde pressure waves, out of all propagating waves, was significantly increased in the last part of phase III (85 +/- 9%, mean, SE), compared with early phase III (6 +/- 5%), late phase II (5 +/- 4%) and the feeding phase (10 +/- 5%), irrespective of gender or previous tube-experience. The median length of the MMCs was 108.5 min. There was no statistically significant difference between men and women or between tube-naive and tube-experienced subjects for the duodeno-jejunal motility indices of phase II and phase III, nor for duration or migration of phase III. The postprandial motility index of the small intestine was increased compared with the interdigestive late phase II, particularly in the jejunum (P < 0.02). The last part of the duodenal interdigestive phase III in healthy subjects shows the feature of a retrosperistaltic pump. This cyclic sequence of retropropagation coincides with the reported rapid alkalinization of the duodenal bulb and the gastric antrum occurring in early antral phase I.