Twenty healthy subjects eating normal diets made repeated five-day stool collections, the 10 females making their collections in four to six successive weeks. In most subjects there were striking variations in transit time, measured by Hinton's method. The variability of average faecal wet and dry weight, faecal volume, and the frequency of defaecation was equally great, suggesting that the transit time variations were genuine. The size of individual stools varied even more, often tenfold or more. Faecal water content was relatively constant. There were no significant differences between males and females, and in the females there were no obvious changes related to the phases of the menstrual cycle. The normal variability of colonic function should be taken into account in planning experiments and in interpreting existing data.