In six healthy mongrel dogs, hypertrophy of the ileum was induced by experimental stenosis. The number, size and morphological appearance of myenteric ganglion cells in the hypertrophic ileal segments were compared with those in corresponding segments of twelve normal control animals. The following observations are reported. 1. The number of ganglion cells is expressed as N/G, i. e. the mean number of neuronal perikarya identifiable in each myenteric ganglion appearing in a complete, 8 μ thick transverse section of the ileum. 2. The mean value of N/G in five control animals was found to be 3.94±0.064, while in hypertrophied intestines a value of 5.17±0.062 was obtained, a difference significant at the P<0.01 level of confidence. 3. The mean diameter of neuronal nuclei in six hypertrophied intestines measured 10.07±0.154 micra, a value significantly higher than the 8.05±0.102 micra mean established in twelve controls. Compared with normal material, the distribution curve of nuclear diameters measured in hypertrophic intestines shows a flattening and a shift to the right. 4. In comparison with normal ganglion cells, most ganglion cells in hypertrophic ilea showed a prominent increase in Nissl substance. 5. In sections impregnated by Jabonero's silver method, evidence was found of an increase in cell-size and in argyrophilia of both argyrophilic and argyrophobic cells in hypertrophied intestines. This phenomenon is particularly pronounced in the larger argentaffine cell groups. The present findings may represent morphological corollaries of changes in the activity level of neurons associated with increased functional demands.