Male rat pups were undernourished in energy and protein by assigning them at birth to lactating dams receiving a low-protein diet (12% casein) to decrease the maternal milk supply. A normal diet (25% casein) was fed to the dams of control pups. On day 21 the pups were killed and hearts were removed and examined from both groups. Hearts from the undernourished animals weighed less and contained less DNA, RNA, and protein. The chemical indices of cell number (DNA) and cell size (wt/DNA) were reduced in the hearts from undernourished animals. Assays of the cardiac tissue for the activity of acetylcholinesterase and butyrocholinesterase indicated that undernutrition altered the activities of both enzymes, but in opposite directions. Protein-energy undernutrition resulted in an increase in acetylcholinesterase activity and a decrease in butyrocholinesterase activity. The specific activities of choline acetyltransferase and cholinesterase were unchanged. The total activity of both cholinesterase and butyrocholinesterase was reduced in the hearts of the undernourished animals. The data indicate that nutritional status can alter the activity of cardiac enzymes which have been associated with controlling the rate and rhythmic contractions of the heart.