The time taken for radio-opaque markers to pass through the intestine has been measured in 25 healthy men, and 18 healthy women in both the follicular and luteal phases of the menstrual cycle. The subjects collected all stools after ingestion of the markers, the number of markers present in each stool was counted on a radiograph, and the number of markers retained in the body was thus determined for 12 hourly intervals after ingestion. The mean values (2 standard deviations) for men and women in both phases of the menstrual cycle proved to be so similar that the results have been combined to provide a single normal range. These data for the normal range for retained markers (as assessed by plain radiograph) are presented in diagrammatic form for clinical use. To assess whether a patient's whole gut transit time lies within the normal range a single type of marker can be used and an abdominal radiograph performed at 12 or 120 hours, the limits of the normal range. Normal subjects retain more than 20% of markers within 12 hours and less than 80% after 120 hours. If desired more information can be gained by giving different types of marker on successive days, so that several transit studies providing intermediate values can be obtained from a single abdominal radiograph at 120 hours.