If the K+ content of normal Ringer solution is increased, the duration of the plateau of the active state begins to rise at 8–10 mM [K]out. If the Cl− of the Ringer solution is replaced by NO3−, the active state duration begins to rise at 4–5 mM [K]out, and increases markedly with further depolarization. The observed dependence of active state duration on [K]out is similar to the known relation between resting metabolism and [K]out and is taken as evidence in support of the concept of subthreshold activation of the excitation–contraction coupling system by depolarization.