In the fasting state, delivery of bile acids into the duodenum is related to the interdigestive migrating motor complex (MMC) and occurs during late phase 2. It has also been shown that in the dog this secretion is transported to the ileum, where bile acids are reabsorbed. We therefore studied plasma bile acid and motilin concentration in relation to small intestinal motility in human volunteers. Motor activity was recorded manometrically in 5 normal subjects after an overnight fast, until at least one full cycle of the migrating complex had been recorded. Pressures were measured in the antrum at 4 levels and in the upper small bowel at 3 levels. Blood samples were drawn every 10 min for radioimmunoassay of motilin and conjugated bile acids. Motilin levels fluctuated in accordance with the MMC cycle. Bile acid levels also fluctuated, but although fluctuations were poorly correlated to motilin levels, peaks of plasma bile acid occurred at or around the time of phase 3 activity. Also, the time lags between bile acid peaks, motilin peaks and phase 3 activity were almost identical, suggesting that peaks of plasma bile acid concentration are due to the arrival and absorption in the ileum of bile acids secreted during the preceding phase 2 and transported by the preceding phase 3. The poor correlation of plasma bile acid peaks with motilin peaks is probably due to the variability of transport and absorption of bile acids.