Guinea-pigs were injected with either 3H-thymidine or colchicine and killed at various time intervals after the injections. Duration of the over-all cell cycle and of the G1, S and G2 phases was determined with autoradiography for cells in the short, medium and long crypts of the colon as well as for the lower, middle and upper segments of the crypt.

The duration of mitosis estimated with the colchicine method increased from the mouth of the crypt downward to the bottom, from about one to three hours. The cells located in the crypt between the mouth and approximately the twentieth cell position have a cell cycle of about 15 hours whereas those located in the crypt section between the twentieth cell position and the bottom have a cell cycle of about 25 hours. This difference is due mainly to a change in the G1 phase. The results of planimetry for the first and second waves of labeled mitoses suggest that in the lower segment of medium and long crypts, cells in the G0 phase occur. It is concluded that the factor(s) controlling cell cycle duration in the colon crypt appears either at the crypt mouth or nearby.