Postnatal development of the sarcolemmal invaginations of right atrial cells of the rat has been studied using standard fixation combined with tannic acid mordanting. T tubules were seen to form at Z lines as simple tubular invaginations starting at the 14th postnatal day. T tubules were present in most cells by the 18th postnatal day but, as in the adult, were restricted to peripheral regions. Also, between the 16th and 18th postnatal day a proliferation of caveolae was seen, both as single vesicles and as complexes with up to a dozen caveolae sharing the same neck. The caveolar complexes persisted in the adult and did not seem to contribute significantly to the formation of the T tubules. Dyadic couplings were seen to become more abundant as T tubules and caveolae proliferated. These findings are discussed in relation to transsarcolemmal Ca2+ movements and excitation-contraction coupling during postnatal development.