Variations in percentage labelling (LI) and mitotic activity (mitoses per crypt) have been studied over a 24-hr period in the epithelial cells and pericryptal fibroblast sheath (PCFS) of the small intestine, caecum, and colon of the mouse. All three tissues displayed clear, synchronized circadian rhythms in DNA-synthetic activity in both the epithelial cells and PCFS. Peak values were coincident within a tissue, but staggered between tissues. In the epithelial cells, peak mitotic values were found between 3 and 6 hr after the peak LI values. The low level of mitotic activity in the PCFS appeared to be synchronized with the rhythms in the adjacent epithelia. The epithelial cells of the lowest crypt third displayed the clearest circadian rhythms. However, the PCFS cells at all levels produced similar curves. A craniocaudal wave of proliferative activity is proposed.