Gastrointestinal transit time was studied by means of the H2 breath test (after administration of 20 g of lactulose) in 20 healthy women with regular menstrual cycles. The mean transit time measured in the follicular phase of the cycle (M +/- SE = 71.25 +/- 6.69 min) did not differ statistically from that measured in the luteal phase (66.75 +/- 5.49 min). Data were statistically analyzed using Student's t test for paired data and the Wilcoxon Test for non-parametric data. Our results show that, in physiological conditions, female sex hormones do not affect mouth-to-caecum transit time. The H2 breath test is easy to perform and well tolerated; it thus proves particularly suitable for use in women of child-bearing age.