The purpose of this experiment was to compare repeated measurements of the gastrointestinal transit times of simultaneously ingested dye and radiopaque pellets. Pellet transit was calculated as the time required for 80% of the dose to be excreted and as mean transit time (MTT-S). Comparisons were made in seven healthy women during three controlled diet periods, two containing different levels of protein and the third added fiber (16 g refined cellulose). Each experimental period was about one month. Dietary protein level or phases of the menstrual cycle had no significant effect on gastrointestinal transit time. Cellulose consumption also did not significantly affect transit time, but it did reduce the variability between repeated 80% transit determinations in four subjects, between MTT-S in three subjects, and between dye in two subjects. The two different measures of pellet transit, 80% pellets and MTT-S, and the dye were significantly correlated, although the correlations were generally stronger during the cellulose diet period. These results suggest that the menstrual cycle, refined cellulose, and dietary protein level have limited effects on gastrointestinal transit time and that measurement of transit as passage of 80% pellets and MTT-S are comparable.