Following a period of fasting, feeding a normal diet results in a burst of DNA synthesis in the crypts of the colonic epithelium. This is due largely to a prompt entry of cells, blocked in G1, into S. Peak levels of S cellularity exceed 4 times the fasting, and 2 times the normal fed control values. Refeeding a low residue diet (soluble casien, glucose and corn oil) results in a return to control levels of proliferative activity, but no hyperplasia. However, in jejunum and ileum, refeeding is followed by a return to near control levels of proliferation with only a slight overshoot in S phase cellularity. During the fasting period, the ileal crypt proliferative compartment (Pc-zone) and total crypt cellularity decline significantly. These changes are accompanied by an increase in the total cycle time, due to an equivalent lengthening of the G1 and S phase. Following refeeding, there is a reduction in the cycle time and a gradual return to the control values for the Pc-zone size and cellularity. In the colon, fasting has no effect on the Pc-zone size or total crypt cellularity. There is an approximate doubling of the cycle time due solely to an increase in G1. Following refeeding there is an increase in the Pc-zone size and crypt cellularity and a marked shortening of the cycle time. Evidence that a G1 cycle blockade is induced in the colon by fasting is given by a lenghening of the G1 period and by stathmokinetic studies employing vincristine.