Immunoreactive rat transforming growth factor-alpha (TGF-alpha) was measured in rat milk, in the mucosa and lumen of the small intestine, and in the pancreas of suckling and adult rats with a homologous RIA. In contrast to epidermal growth factor, where the main source of epidermal growth factor for sucklings is rat milk, the presence of TGF-alpha was not detectable in rat milk. The concentrations of TGF-alpha in the small intestine exhibited similar values in suckling and adult rats, whereas epidermal growth factor levels in the small intestine were several times higher in suckling rats than adults. Overnight fasting in suckling rats resulted in minimal changes in the luminal and mucosal TGF-alpha content. A positive correlation was established in suckling rats between the TGF-alpha content in the intestinal lumen (but not mucosa) and the TGF-alpha content in pancreas. Despite many structural and functional similarities between TGF-alpha and epidermal growth factor, our present data indicate significant differences in the origin and distribution of these two growth factors in the tract of the small intestine of developing rats.