Isometric contraction of cat spleen capsule due to noradrenaline had a fast and a slow phase. These phases were due to a tightly and a loosely bound (or extracellular) pool of calcium, respectively. Depolarization due to high potassium caused mainly a reduction of the slow phase. In sodium-loaded spleen strips recovering from potassium deprivation, the sodium pump is electrogenic and the muscle cells are probably hyperpolarized. In this situation also the slow phase of noradrenaline response is reduced. It is concluded that both depolarization and hyperpolarization affect the utilization of loosely bound calcium and that tightly bound calcium may be released during 'pharmacomechanical coupling' in splenic smooth muscle.