The time course of the latency relaxation was studied at various temperatures in the range 0-26 degrees. Over the entire range the time of onset of the drop in tension, t1, was independent of sarcomere length. At temperatures above 12-15 degrees C the falling phase had a point of inflexion, while at lower temperatures there was an interval during which the tension fell at a constant rate. The time when the rate of drop in tension had passed its maximum value tL2, the time to the maximum drop in tension t2, and the time when the tension crossed the resting level t3, all showed linear dependence on sarcomere length in the range from 2.1 to 2.7-3.4 microgram. In this range the durations of the intervals tL2-t1, t2-t1, and t3-t1 were nearly proportional to the distance from the Z-line to the end of the zone of overlap between the thick and the thin filaments. This could be explained as the activation being a longitudinal process starting from the Z-line. The slopes (dt/dS) of the linear portions of the time variables t1.2, t2, and t3 in a time-sarcomere length (S) diagram all had the same dependence on temperature giving a Q10 of 1.75. Under the assumption that the activation process followed a diffusion of calcium from the Z-line region to the zone of overlap a diffusion coefficient was estimated. At room temperature it had a magnitude of about 1/20 of that for calcium chloride in water. It had a dependence on temperature corresponding to an Arrhenius activation energy of about 37 kJ/mol which is about twice the activation energy for a simple diffusion of calcium in water. The results can be interpreted in terms of the time course of the latency relaxation mainly reflecting a longitudinal diffusion of calcium ions in the sarcoplasm.