The heights of villi were less in all the segments of mildly undernourished (by increasing litter size) pups at 21 days of age, in comparison with controls, while the heights of villi in the duodenum and jejunum, but not in the ileum, were less in severely undernourished (by maternal protein deficiency) pups. Administration of hydrocortisone during 18–21 days of age to normal pups and to pups undernourished by maternal protein deficiency during lactation showed a marked reduction of villi heights in the ileum but not in other segments of the intestine. When the pups of protein-deficient mothers (during lactation) were fed on a normal diet during the post-weaning period, the villi in the ileum were found to be longer than those of controls. Superimposition of post-weaning protein deficiency over pre-weaning undernutrition only left the heights of ileal villi unaltered, but in other segments deficits in heights of villi were observed. These studies suggest that the suggested ‘villi-reducing factor’ did not appear in the ileum of severely undernourished pups even after the availability of an adequate nutrition in the post-weaning period, probably due to hydrocortisone deficiency at about 21 days of age, and that the hormone has a role in maintaining ileal villi heights. In addition to the heights of villi, other morphological retardations and alterations of enterocyte structure were observed in the severely undernourished intestine. These effects differed in different parts of the intestine and with the degree of neonatal undernutrition. Most of the morphological abnormalities showed a trend for recovery when a normal diet was given in the post-weaning p