Single crab (Callinectes danae) fibers were equilibrated with isotonic, high KC1 solutions and were subsequently returned to the control saline. This caused marked swelling of the T tubules. Fibers treated with 100 mMKCl had a 2.5-mV residual depolarization, a 50% decrease in effective membrane resistance (Reff) and a 75% reduction in membrane time constant (τm). These fibers exhibited large increases in membrane conductance upon depolarization and were inexcitable; membrane depolarization with current pulses elicited no contraction. The effects of the KCl treatment on membrane properties were not reproduced by treatment with high potassium gluconate solutions, which did not cause tubular swelling. Tetrabutylammonium (10 mM) or Ba ions (10–20 mM), but not tetraethylammonium (40–100 mM), Sr ions (15–70 mM), or procaine (1–8 mM) reversed the effects of the KCl treatment onReff, τm, membrane excitability, and excitation–contraction coupling. The time course of the Ba effects was consistent with the suggestion that the KCl treatment increases the K conductance of the tubular membranes, which in turn prevents the activation of voltage-dependent Ca channels located in the membranes of the T system. This results in inhibition of the Ca-dependent electrogenesis and consequently, the absence of contraction upon depolarization of the plasma membrane.