The circadian rhythm of the rectal cell proliferation was studied in five patients affected by advanced colon cancer. Biopsies were taken from apparently normal mucosa at 10 cm from the anal verge, every 6 h in a 24-h period. Fragments were incubated for 1 h in a culture medium containing bromodeoxyuridine (BrdUrd). As compared with the mean 24 h values, the percentage of BrdUrd-labelled cells in the crypts (Labelling Index, LI) was lower in the specimens collected at 10.00 PM (P = 0.02) The LI in such biopsies was also lower than the LI observed at the baseline time, 10.00 AM (P = 0.001) The results suggest that the rectal cell proliferation in patients with advanced colon cancer fluctuates during the day. The study of the rhythmicity of the intestinal cells may be useful to modulate the infusion of antiproliferative agents to prevent damage of the normal colorectal mucosa.