The t-tubules are invaginations of the surface membrane of mammalian ventricular myocytes. The majority of trans-sarcolemmal Ca flux occurs across the t-tubule membrane (Brette & Orchard, 2003); however Ca within the t-tubules does not equilibrate instantaneously with the bulk extracellular solution (Blatter & Niggli, 1998). Thus during activity, [Ca] may change within the t-tubule lumen; this would, in turn, alter trans-sarcolemmal Ca flux. To test this hypothesis, we used a computer model of the ventricular myocyte including a t-tubular system (Pasek et al. 2003), modified to be consistent with data from rat ventricular myocytes. The t-tubules are described as a single compartment separated from the bulk extracellular solution by the mean resistance of the tubular system. A single t-tubule is represented as a cylindrical conductor with a lumen resistance of 9.8 MΩ. The fraction of membrane within the t-tubule compartment (32 %) and the distribution of ion transport mechanisms between the surface and t-tubule membranes were set as determined using detubulation of rat ventricular myocytes (Brette & Orchard, 2003). The action potentials in the two membranes were not significantly different. However, [Ca] within the t-tubule lumen decreased on each stimulus, and decreased cumulatively, until it reached a new dynamic steady state, with increasing stimulation rate (Fig. 1). Figure 1 also shows that Ca depletion in the t-tubule lumen was reflected in decreased sarcoplasmic reticulum Ca content and intracellular [Ca] transient amplitude (solid lines), in comparison with a ventricular cell model that did not take the t-tubules into account (dotted lines), but used the same total cell membrane capacitance and current magnitudes. These data suggest that activity-dependent depletion of Ca within the t-tubule lumen, adjacent to the trans-sarcolemmal Ca flux pathways, may decrease the Ca load, and hence the inotropic status, of ventricular myocytes in physiological conditions.