Abstract Leptomeric fibrils are studied electron microscopically in Purkinje cells of the hen and pigeon heart and in ordinary myocardial cells of the hen. The leptomeric fibrils lie chiefly subjacent to the sarcolemma and are often related to sarcolemmal invaginations. They are frequently attached to normomeric myofibrils. Leptomeric fibrils are found preferentially in connective tissue embedded Purkinje cells. In the Purkinje cells of the hen heart a special leptomeric structure—the leptomeric complex—is observed. The leptomeric complexes are often crossed by normomeric myofibrils. Lysosomes and lamellated bodies occur in the neighbourhood and in the interior of leptomeric complexes. Possible functions of the leptomeric fibrils and leptomeric complexes as suggested in the literature are reviewed and discussed in the light of the present findings.