It has been suggested that in mammalian atrial cells, which lack T-tubules, depolarisation triggers a Ca2+ rise in the subsarcolemmal region, followed by a variable degree of Ca2+ signal propagation into deeper myofibril layers. Measuring isometric force in mouse atrial and ventricular trabeculae, we test the hypothesis that myofibril layer recruitment can provide an additional mechanism by which atrial but not ventricular myocytes modulate contractile force. 1) In atrial but not ventricular trabaculae twitch force amplitude transitorily increases after an abrupt reduction of intracellular Ca2+ buffering obtained with an SR-Ca-pump blocker. 2) Contractile force rises much more in atrial than ventricular trabeculae on increase external [Ca2+]. These effects in atrial myocardium could be due to increased inward spread of Ca2+ and recruitment of myofibril layers by reduction of Ca2+ buffering or increased extracellular Ca2+ influx respectively. In Atrial myocytes inward Ca2+ spread may result in a temporal delay in activation of the deeper myofibril layers and may therefore prolong contraction. In evidence: 1) isometric twitch duration is prolonged in atrial but not ventricular trabeculae on increase in extracellular [Ca2+]; 2) increase in contraction kinetics in response to high stimulation frequency or cathecolamines is markedly reduced in atrial compared to ventricular tissue.