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1. Junctional potentials were recorded with a micro-electrode inserted into muscle fibres of the rectus abdominis muscle of the frog. Large and small nerve fibres were stimulated separately, using a selective stimulation technique. In a few muscle fibres rectangular current pulses were applied through a second micro-electrode to examine current—voltage relations.

2. Two groups of muscle fibres could be distinguished: (a) Muscle fibres with resting potentials less negative than -75 mV, being localized at the ventral surface only and representing about 9% of the superficial fibres. These fibres were found to be innervated by small (high-threshold) nerve fibres and responded to single indirect stimuli with a typical s.j.p. (long latency, multiple components, after-hyperpolarization); upon repetitive stimulation summation of s.j.p.s and a slowly increasing tension could be recorded. The fibres showed a slow time course of electrotonic potentials, relatively high membrane resistance and `delayed rectification'. (b) Muscle fibres with generally much higher resting potentials, being innervated by large (low-threshold) nerve fibres only. Upon indirect stimulation a short-latency end-plate or action potential (followed by a twitch) was recorded. The membrane time constant and input resistance of these fibres were similar to those of twitch fibres in other frog muscles.

3. No evidence was found in the present work for the existence of an intermediate type of muscle fibre.

4. It is concluded that the rectus muscle, similar to other frog muscles, contains two distinct types of muscle fibres, twitch and slow.