The purpose of this study was to assess the jejunal and ileal brush border injury caused by Yersinia enterocolitica and to correlate these alterations with functional abnormalities. Weanling rabbits infected with 10(10) organisms of a human pathogenic Y. enterocolitica strain were compared with control and pair-fed, sham-treated animals. On day 6, infection resulted in a diffuse decrease in brush border enzyme activities in the small intestine and villus atrophy and crypt hyperplasia in the ileum. By day 14, ileal architecture and jejunal disaccharidases had returned to normal, but enzyme abnormalities persisted in the ileum. Ultrastructural studies showed decreased brush border surface area in the jejunum and ileum on day 6 and in the ileum on day 14 of infection. Abnormalities of brush border function caused by infection correlated with the changes in microvillus surface area. In pair-fed animals on day 6, brush border surface area was slightly decreased in the ileum but increased in the jejunum, suggesting that the brush border injury resulted from infection rather than from malnutrition alone. The findings indicate that Y. enterocolitica inflicts a diffuse brush border injury that is in keeping with the generalized defect in brush border enzyme activity and transport function.