The present study was planned to observe the effect of protein-energy malnutrition on the gastric and duodenal mucosa. The activities of digestive enzymes (i.e. lactase, sucrase, maltase, trehalase, glucoamylase, leucine aminopeptidase, alkaline phosphatase and gamma-glutamyl transpeptidase) from the gastric (fundus, body and antrum) and duodenal mucosa [i.e. first (D1) and second (D2) part of the duodenum] were determined in 6 control, 6 protein-energy malnourished (PEM) and 6 rehabilitated young rhesus monkeys. There was a significant increase in the activity of the lactase enzyme in the antrum, and D1 and D2 portions of the duodenum of PEM monkeys, while the activity of all other enzymes was significantly increased in the D1 and D2 portions only. The increase in the activity of the above-mentioned enzymes became normal upon rehabilitation. There was no change in the enzyme activities of the gastric mucosa in mild-to-moderate PEM states. This study demonstrates that even mild-to-moderate malnutrition states affect the activity of enzymes in the gastric and duodenal mucosa. Enzyme activity recovers on rehabilitation.