Choline acetyltransferase is the enzyme that catalyzes the biosynthesis of acetylcholine, the neurotransmitter of the pre- and postganglionic parasympathetic system. To assess the extent of parasympathetic innervation, enzyme activity was measured in specialized and contractile regions throughout the guinea pig heart. Enzyme activity in the right atrial appendage was 137 nmol g(-1) hr(-1). Activity was greatest in the region of the sinoatrial node (187 nmol g(-1) hr(-1). Also, enzyme activity was high in the regions of the atrioventricular node (153 nmol g(-1) hr(-1), the proximal conduction bundles (133 nmol g(-1) hr(-1), and the base of the anterior papillary muscle of the right ventricle (179 nmol g(-1) hr(-1), which contains the moderator band and Purkinje fibers. In contrast, the enzyme activity in the inferior interventricular septum and the free walls of the right and left ventricles, which are more predominantly contractile tissue, was 67 +/- 6,108 +/- 14, and 56 +/- 11 nmol g(-1) hr(-1), respectively. This activity is significantly lower than in the right atrial appendage. These results suggest that the density of parasympathetic innervation is similar in all the components of the conduction system, from the sinoatrial node to Purkinje tissues. Furthermore, the parasympathetic innervation of regions specialized for conduction is up to four times more dense than that of contractile regions.