The effects of gastroenteroanastomosis, antral exclusion and antral resection on the enterochromaffin (argentaffin) cell system in the rat alimentary tract were studied 6 weeks after the operation. When measured as cell density per mm3 mucosa, the number of enterochromaffin cells was decreased in most gastrointestinal regions following the three surgical procedures. In the antral exclusion group this decrease was most apparent in the caecum and large intestine, and in the animals with antral resection it was most pronounced in the proximal and middle regions of the small intestine. When expressed per segment mucosa, the enterochromaffin cells were even more reduced in number in all three experimental groups, as a result of mucosal atrophy in the entire gastrointestinal tract. The atrophy was most pronounced in animals with gastroenteroanastomosis, and least pronounced in those with antral exclusion. The underlying mechanisms of the effects on the enterochromaffin cell system under these experimental conditions are discussed.