To determine the nature and duration of colostrum-induced enteric mucosal growth, jejunal mucosal weight as well as protein, DNA and RNA content of beagle puppies were determined at birth and after 24, 72 and 120 h of either natural (suckling) or formula (Esbilac) feeding. Mucosal hydrolase activities of all animals were also determined. Jejunal mucosal weight, protein, DNA and RNA of suckled animals were 80%, 200%, 200% and 200% greater at 24 h than at birth (p<0.05); values at 72 and 120 h were similar to those at 24 h. In formula-fed animals, neither jejunal mass, protein, DNA nor RNA at 24 and 72 h was different from birth; at 120 h, however, all were similar to those of suckled animals. Lactase specific activity of suckled animals at 24 and 72 h was only 67% that at birth (p<0.05) whereas sucrase, leucine aminopeptidase and alkaline phosphatase activities were not different. In formula-fed animals, enzyme activities at 24 and 72 h were similar to those at birth; at 120 h, however, lactase and alkaline phosphatase activities were lower (p<0.05). These data confirm earlier studies showing that colostrum stimulates hyperplasia of enteric mucosa. They demonstrate also that this effect is limited to the first 24 h of life and that formula feeding exerts a similar effect but only after at least 72 h. The hydrolase data suggest that this rapid mucosal proliferation results in a relatively immature villus cell population.