Subcutaneous octreotide (Sandostatin) injections lead to gall stone formation in 13-50% of acromegaly patients during one year of therapy. This study explored the effects of octreotide on interdigestive gall bladder emptying, antroduodenal motility, and motilin release. Ambulatory antroduodenal manometry was performed in six acromegaly patients before and after two months of octreotide therapy (100 micrograms thrice daily, subcutaneously). Ultrasonographic gall bladder volume measurements and plasma motilin concentrations were obtained during two migrating motor complex (MMC) cycles. Before octreotide treatment, nine of 26 phase III activities started in the antrum and 17 of 26 in the duodenum whereas during treatment 47 of 48 of phase III activity started in the duodenum (p < 0.05). Before treatment, interdigestive gall bladder emptying (mean (SEM) 39.9 (4.0)% of maximal fasting volume) and plasma motilin peaks preceded antral phase III but not duodenal phase III. During octreotide therapy no significant motilin fluctuation or gall bladder emptying was seen. Fasting gall bladder volume increased from 40.9 (9.1) ml before to 68.0 (14.8) ml (p < 0.05) during octreotide treatment. In conclusion, two months' treatment with octreotide increases the number of duodenal phase III like activity and virtually abolishes antral phase III, plasma motilin peaks, and interdigestive gall bladder emptying. These effects might contribute to the high risk of gall stone formation during longterm octreotide treatment.