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The authors examined, using histochemical methods, the AV conduction system in five infants and two children who died unexpectedly with a congenital heart disease. The activity of "glycogen dependent" phosphorylase in the AV conduction system was mostly negative or weak. The probable cause of this finding is hypoxia, although the effect of hypotrophy must be admitted. In groups of cells penetrating into connective septal tissue a decrease of oxidoreductases and hydrolases was found. These findings may be associated with regressive changes in the postnatal reduction of the AV conduction system.