Ca indicator dye arsenazo III (AZ III) was injected into Xenopus skeletal muscle fibers. AZ III Ca-transients (AZ III signals) responding to the electrical stimulation of the fibers were measured by a pair of photomultipliers at 651-721 nm. In the standard Ringer solution at 22-23 degrees C, the latency of the AZ III signal (T1), the time from the onset of signal to the peak (T2), and the half decay time (T3) were 1.5, 3.9, and 16.6 msec, respectively. The falling phase of the signal was fitted to the exponential curve. The effects of repetitive stimulation with frequency of 10, 20, and 30/sec were investigated. The AZ III signal at 30/sec showed obvious summation. Prolongation of the falling phase with an increase in stimulating pulses or in stimulus frequency was always observed. In the solution which contained twitch potentiators such as NO3-, Zn2+, and caffeine, the peak of the AZ III signal showed a 10-50% increase and T1, T2, and T3 were increased by 20-100%. The falling phase of the AZ III signal was convex on the logarithmic scale in NO3 or Zn solution, and fitted to the exponential curve in caffeine solution. The factors which determined the falling phase are discussed.