Malnutrition was induced in the immediate postnatal period by expanding newborn litters to 20 rat pups/dam. The reversibility of the effects of malnutrition on the pancreas was evaluated by comparing two different feeding methods. At 21 days of age, pups from the expanded litters exhibited significantly decreased body (P less than 0.0005) and pancreatic (P less than 0.0025) weights as compared to those from control litters (12 pups/dam). Malnourished pups also had less contents of amylase (P less than 0.01), lipase (P less than 0.0005) and trypsinogen (P less than 0.0025) in their pancreases. The concentrations (specific activities) of amylase (P less than 0.05) and lipase (P less than 0.0125) were significantly decreased but trypsinogen (P less than 0.35) was not affected. Subsequent nutritional rehabilitation by an ad libitum (food available 24 h/day) or restricted (food available 2 h/day) feeding regimen failed to allow for "catch-up" in body (P less than 0.025) and pancreatic weight (P less than 0.05) by 56 days of life. With ad libitum feedings, enzyme contents and concentrations of amylase and lipase in malnourished animals attained control values by 7 and 14 days, respectively. Restricted feedings, however, delayed the recovery in amylase by an additional 7 days but lipase remained depressed in both content, (P less than 0.005) and specific activity (P less than 0.0025) for the duration of the experiment (56 days). Changes in pancreatic enzymes in response to malnutrition are readily reversible with ad libitum feedings but changes in somatic and pancreatic weights were not reversed.(ABSTRACT TRUNCATED AT 250 WORDS)