Gastrointestinal transit time as well as serum estradiol and progesterone levels were measured in 15 normally menstruating women twice during their menstrual cycle, once in the follicular phase (days 8-10) when progesterone levels are low and once in the luteal phase (days 18-20) when progesterone levels are increased. Each subject had a progesterone rise during the luteal phase and onset of menses at the expected time documenting ovulatory cycles. Gastrointestinal transit time from ingestion of lactulose to the delivery of the disaccharide to the cecum was determined by monitoring breath hydrogen levels at 10-min intervals. Gastrointestinal transit time was significantly (p less than 0.01) prolonged in the luteal phase when progesterone levels were increased compared with the follicular phase. This study demonstrates that the menstrual cycle plays an important role in determining the gastrointestinal transit time in normally menstruating women.