The photoprotein aequorin was micro-injected into several cells in a sheep Purkinje fibre. The intracellular Ca concentration [( Ca2+]i) was measured from the resulting light emission. Inhibition of the Na-K pump with strophanthidin resulted in the development of tonic tension which increased on depolarization. This increase was accompanied by an increase of aequorin light. Increasing external Ca concentration [( Ca2+]o) or the magnitude of the depolarization increased both light and tension. If the depolarizing pulse was maintained for several minutes then both tonic tension and aequorin light slowly decayed. The relationship between tension and light was unaffected during this decay. On repolarization the light decayed to below the level before the depolarization before slowly increasing. During this period a test depolarization produced increases of aequorin light and tension which were smaller than control. The application of ryanodine (1-10 microM) abolished all components of tension other than the tonic component. Under these conditions the time course of the increase of tonic tension and aequorin light on depolarization was sufficiently slow to be measured. In most (five out of six) experiments the relationship between light and tension during this development of tonic tension was found to be similar to that during the subsequent spontaneous decay. However, in one experiment the decay of force was greater than could be accounted for by the fall of [Ca2+]i. It is concluded that most of the spontaneous relaxation of tonic tension can be attributed to a fall of [Ca2+]i rather than to other explanations such as an intracellular acidification or increase of inorganic phosphate concentration.