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1. Intracellular changes in free Ca2+ concentration were recorded from slow muscle fibres in the pyriformis muscle of Rana temporaria, using the dye arsenazo III. Fibres were voltage clamped, and arsenazo signals were recorded in response to depolarizing pulses. 2. The size of the arsenazo response to depolarizing pulses of 100 msec duration was a sigmoid function of membrane potential over the range -45 to 0 mV, and remained constant with further depolarizations up to +100 mV. 3. The peak size of the arsenazo signal to supramaximal depolarizations increased with increasing pulse length. The initial rising phase during a pulse was much slower than in twitch fibres, and this phase was followed by an even slower rise. Following short pulses the decay of the response was exponential, with a time constant of about 1.4 sec, while after long pulses the decline became much slower. 4. Decreasing free Ca2+ concentration in the bathing medium to very low levels, using EGTA , did not affect the responses to short (100 msec) depolarizations. 5. Slow fibres bathed in Ringer's solution containing 12 mM-Ca2+ showed a well maintained arsenazo response to supramaximal depolarizations lasting over 1 min. Reduction of external Ca2+ to 1.8 and (nominally) 0 mM caused the response to become progressively more transient. 6. After denervation, slow fibres developed action potentials, but non of the parameters of the arsenazo response was significantly changed. During the early phase of reinnervation by a mixed nerve, when fast conduction axons begin to innervate slow fibres, the ability to give a maintained response during long depolarizations was reduced. 7. It is concluded that intracellular Ca2+ transients in slow muscle fibres are probably generated by a similar mechanism as in twitch fibres and entry of external Ca2+ is not an appreciable factor. The slow time course of the transients may be important in determining the time courses of tension development and relaxation.