Tritiated thymidine was administered IP to rats that had been exposed to benzalkonium chloride in the duodenum, jejunum, and ileum, resulting in neuronal ablation. Epithelial cell proliferation and migration were studied 21 and 7 days after treatment. Significant hyperplasia and hypertrophy of the villi and crypts was seen from day 7 on. This was half as pronounced as that of the muscle layer, whose maximal percent increase was not seen until day 21. In the crypt, the proliferation had increased significantly (65% 3H index corrected) and its zone had expanded proportionally to the total crypt depth. After an average of 36 hours in the ileum (48 hours in normal rats), labeled cells reached the tip of the lengthened villi, reflecting significantly accelerated migration. Concerning the distributional pattern of the labeled cells in the crypt, a nonsignificant shift to the lower two thirds of the crypt could be distinguished. From this the author concludes that treatment with benzalkonium chloride influences the proliferation and migration of the epithelial cells in the treated area. These alterations may result from loss of the myenteric plexus, but other factors cannot be excluded.