This study was undertaken to establish if there is a correlation between the ontogenesis of cholinesterase activities and the cholinergic muscarinic receptors in the development of rat pancreas. Two different cholinesterase activities were detected in the pancreas: a true acetylcholinesterase and a pseudocholinesterase. The ratio of acetylcholinesterase to total cholinesterase activities varied as a function of age. Acetylcholinesterase and total cholinesterase specific activities (units per milligram protein) decreased sharply at weaning. When expressed on a DNA basis, both activities decreased constantly until age 30 days to increase slightly thereafter. However, a correlation could not be established between the development of acetylcholinesterase activity and the muscarinic receptors.