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1. External and micro-electrode measurements were made of fibre membrane potentials taken at intervals along the length of frog sartorius muscle fibres before and after adding choline. In `cold-treated' Rana pipiens which had been kept at 1-3° C for at least 5 days, 10^-3 w/v choline caused a long-lasting depolarization of approximately 3 hr. Depolarization occurred first at the end-plate region and spread more slowly to the end-plate free region. The potential became equalized along the muscles during recovery.

2. R. pipiens kept at room temperature showed a faster repolarization in the choline solution, within approximately 1 hr. R. temporaria showed a similar recovery time in muscles from `cold-treated' animals. In R. temporaria kept at room temperature the duration was shorter, approximately 20-30 min. The apparently generalized depolarization of the muscle seen in `cold-treated' R. pipiens appears to be due to the spreading effect of a long-maintained depolarization at the end-plates. A similar far-spread of depolarization was seen following crush of fibres.

3. Length-constants of muscle fibres determined with pulses were found to be within the normal range in muscles from `cold-treated' R. pipiens and slightly reduced after adding choline.