Sprague-Dawley rats fed a nonpurified diet from weaning to 3 mo (young) or 12 mo (middle aged) of age were fed a low (5 cal%) starch (LST) purified diet for 2 wk. They were then fed a high (70 cal%) starch (HST) purified diet for up to 4 wk. Body weights of both age groups were greater in rats fed the LST than in those fed nonpurified diet throughout. The young, but not the middle-aged, group continued this gain while consuming HST. The levels of activity of intestinal disaccharidases of upper (proximal one-third) and lower (middle one-third) jejunum, and pancreatic alpha-amylase were lower in rats fed LST diet in both age groups than in rats fed HST. Similar increases of specific (per protein or DNA) activity were observed in both age groups throughout the first three (disaccharidase) to four (pancreatic alpha-amylase) days of feeding HST. Values of specific activity of the middle-aged group returned to initial levels with continued feeding of HST. In contrast, values in the young group tended to plateau (disaccharidases) or continued to increase (alpha-amylase). Differences in adaptability over 1 mo were most dramatic for alpha-amylase and sucrase, but negligible for lactase.