In order to characterize the response of the pancreas to undernutrition during the critical neonatal growth phase, acquired postnatal malnutrition was induced in the rat, using the expanded litter. An experimental nursing litter of 16 rats and control litters of 7 to 8 rats were formed. At 19 days of age, the pups were killed. Mean pancreatic wet weight was decreased in the malnourished rat to a greater extent than the decrease in total body weight (49 versus 60%). Decreased organ weight was predominantly the result of a decrease in DNA content and cell number. Enzyme activities expressed per total organ were all diminished; lipase to the greatest extent; trypsin and amylase to an intermediate extent; followed by chymotrypsin and the carboxypeptidases. The specific activities of lipase and trypsin were decreased with lipase, the most severely effected. The low trypsin levels can be attributed to trypsin inhibitor. It is possible therefore, that only the specific activity of lipase is significantly decreased. The decrease in enzyme activities, expressed both as specific activities and as total organ activities were decreased in a nonparallel fashion.