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Transverse (t) tubules are surface membrane invaginations that are present in all mammalian cardiac ventricular cells. The apposition of L-type Ca²⁺ channels on t tubules with the sarcoplasmic reticulum (SR) constitutes a “calcium release unit” and allows close coupling of excitation to the rise in systolic Ca²⁺. T tubules are virtually absent in the atria of small mammals, and therefore Ca²⁺ release from the SR occurs initially at the periphery of the cell and then propagates into the interior. Recent work has, however, shown the occurrence of t tubules in atrial myocytes from sheep. As in the ventricle, Ca²⁺ release in these cells occurs simultaneously in central and peripheral regions. T tubules in both the atria and the ventricle are lost in disease, contributing to cellular dysfunction. The aim of this study was to determine if the occurrence of t tubules in the atrium is restricted to sheep or is a more general property of …