The objectives of the study were to determine whether the follicular (F; days 6-11) and luteal (L; days 16-21) phases of the menstrual cycle were associated with changes in starch malabsorption, stool bulking, stool mucinase, and beta-glucuronidase activities in 10 women (24.1 +/- 0.7 years old) eating a standardized low-fibre diet. Starch malabsorption, measured by breath hydrogen excretion after a breakfast of pureed chickpea (days 10 and 20) versus 10 g lactulose (days 11 and 21), decreased from 9.7 +/- 1.8 g/50 g starch ingested (F) to 6.6 +/- 1.8 g/50 g starch ingested (L) (P less than 0.05). Stool wet weight decreased from 84.5 +/- 10.1 g/day (F) to 52.2 +/- 5.8 g/day (L) (P less than 0.002). Stool dry weight decreased from 20.2 +/- 1.9 g/day (F) to 14.2 +/- 1.1 g/day (L) (P less than 0.006). Stool nitrogen excretion decreased from 1.81 +/- 0.19 g/day (F) to 0.82 +/- 0.06 g/day (L) (P less than 0.006). Stool mucinase and beta-glucuronidase activities were unaffected by the menstrual cycle. These results indicate that women eating low-fibre Western diets may be more prone to constipation during the luteal phase of the menstrual cycle.