The present study was designed to analyze the effect of guanethidine-induced sympathectomy on ileal crypt cell proliferation before neonatal closure to macromolecular absorption (pre-closure ileum). The present experiments demonstrated an alteration of the crypt cell generation cycle (T<sub>C</sub>) in the pre-closure ileum (15 days after birth) with guanethidine treatment. Percent labeled mitoses measurements for 15-day-old rats indicated a lengthening of T<sub>C</sub> from 18.1 to 23.0 h with the elongation primarily due to alteration of T<sub>G1</sub> (pre-DNA-synthetic gap period) and T<sub>S</sub> (DNA-synthetic period). Mitotic and labeling indices and the amplitude of the circadian rhythm in mitotic activity were significantly depressed in guanethidine-treated rats.