A quantitative light microscopic study has been carried out on the myenteric and submucosal ganglia of the guinea-pig ileum, after inducing hypertrophy of the wall with an experimental stenosis. The area of the profiles of nerve cells, of nerve cell nuclei and of glial nuclei, and the percentage area of the neuropil were measured, and the relative numbers of neurons and glial cells were estimated. The average size of neurons in both plexuses was greatly increased over the control values. Nearly half of the neuronal profiles measured over 500 microns2 in sectional area (less than 3% in the controls), and less than 1% measured less than 150 microns2 (21% in controls). The average size of glial nuclei (and presumably that of the glial cells too) was increased in the hypertrophic ganglia. The number of glial cells (relative to the number of neurons) was unchanged or slightly decreased in the hypertrophic myenteric ganglia; in contrast, it was markedly increased in the submucosal ganglia. In both ganglia the hypertrophy was accompanied by a decrease in the percentage volume of neuropil.