In order to study cardiac myocyte development different approaches were established during the last decades. The main purpose of these studies was the differentiation of cardiac precursor cells into specialized, differentiated cell types, as well as the development of functional properties such as Ca2q handling, rhythm generation and excitationcontraction coupling of cardiomyocytes during development. Although considerable data exist about skeletal myowx genesis 1–3, limited knowledge is available with regard to the origin of the commitment and differentiation of cardiac cells. A comprehensive, morphological study on the cytodifferentiation from mesenchymal cells into cardiac myocytes is described in the embryonic murine heart w x4: According to the authors, different stages of myofibrillogenesis are present during embryological myocardial development. Cells with no or only little myofibrillar arrangement develop to myocardial cells with wx orientated myofibrils 5, 6. A number of morphological studies have investigated heart development on embryonic, neonatal and adult isolated cardiomyocytes also from difw x ferent species 7–16. Although the ultrastructure during cardiac development wx has been thoroughly investigated 17, still relatively little is known on the development of excitability of the mam-Ž. malian heart, most importantly: 1: The relation between Ž w x. expression of cardio-specific genes see review 23, the formation of cardiac phenotypes and the functional expres-Ž. sion of different types of ion channels; 2: The regulation Ž and genetic control of expression of ion channels eg by