Migration velocity estimates have been determined at each position along the crypt length for both the small and large intestine of the mouse at 6 different times of the day. Measurements also have been made of crypt circumference and length. Dramatic, and significant (P < 0.001), changes in migration velocity as a function of time of day were observed in the small intestine with a maximum 0.84 cell positions (cp) per hour at 0900 h and a minimum of -0.46 cp/h at 1700 h, although the negative velocity was probably artefactual. The 24-h mean velocity rose smoothly as a function of cell position to a peak of 0.45 cp/h at cell position 17 (around the top of the proliferative zone). Much more modest changes were seen in the percent of 3HTdR labelled cells (minimum 30.8%, maximum 38.3%, P < 0.001) and crypt circumference (minimum 16.9 cells, maximum 17.9 cells, P = 0.003). The migration velocity was rather less well determined in the large intestine with a peak in the 24-h mean velocity (0.26 cp/h) occurring at cell position 10. At this position significant circadian variation was detected (minimum -0.39 cp/h, maximum 0.75 cp/h, P = 0.006). Changes were seen in the percent of labelled cells (minimum 9.4%, maximum 22.3%, P < 0.001) and crypt circumference (minimum 18.3 cells, maximum 19.2 cells, P < 0.001). In both tissues it is suggested that the combination of the modest changes in cell proliferation rates in conjunction with the changes in crypt cell number can account for the large amplitude in variation of crypt output, and that the reservoir effects of changes in crypt geometry are an essential part of the process governing the maintenance of intestinal cell numbers.