After birth, the gastrointestinal tract of the neonate is exposed to food and bacterial and environmental antigens. Maternal milk components may play a role in regulation of mucosal immune activity to luminal antigens. In this study we determine the ontogeny of transforming growth factor (TGF)-beta1-producing cells in the rat pup small intestine and assess maternal milk concentrations of TGF-beta. Intestinal tissue samples of duodenum and ileum were collected, processed, and stained for TGF-beta1, and in situ hybridization for TGF-beta1 mRNA was also performed on the duodenum. TGF-beta levels in milk were assayed by ELISA. TGF-beta2 levels in milk were high at d 6, and declined thereafter at d 10 and 19. TGF-beta1 was not detected. In contrast, the cell number and intensity of staining of TGF-beta1 peptide in the small intestine was low in 3- and 10-d-old rats and increased markedly by 19 d of life. In the duodenum mRNA levels mirrored this trend. TGF-beta1 expression in the lamina propria was absent before d 19, and increased progressively over time. Maternal milk TGF-beta2 levels are high in early milk and decrease during the weaning period. In contrast, endogenous TGF-beta production in the small intestine increases during the weaning period.