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1. The membrane properties of the circular muscle cells of guinea-pig caecum and nervous factors influencing the muscle activity were studied with micro-electrodes using partition and field stimulating methods.

2. The mean membrane potential was -52 mV. Spontaneous discharges appeared as regular bursts between silent periods, as regular spikes without silent period, or as regular slow potential changes with superimposed spikes.

3. Spontaneous spikes with overshoot were frequently observed. The mean maximum rate of rise was 5·2 V/sec. The mean conduction velocity of evoked spikes was 5·4 cm/sec.

4. The amplitude of the elctrotonic potential was linearly proportional to the current applied by the partition stimulating method. The spatial decay of the electrotonic potential along the tissue was exponential, with a mean length constant of 1·7 mm.

5. The time constants of the membrane calculated from the electrotonic potential, and from the conduction velocity, length constant and time course of the foot of the spike were about 200 and 100 msec respectively. These results indicate that the circular muscle of guinea-pig caecum possesses cable like properties.

6. Field stimulation (0·3 msec pulse duration) to the circular muscle evoked three different responses successively, i.e. initial depolarization (initial excitatory junction potential) with or without spike, hyperpolarization (inhibitory junction potential) and delayed depolarization (delayed excitatory junction potential) with or without spikes.

7. These three different potential changes were completely blocked by treatment with tetrodotoxin (5 x 10^-6 g/ml.), and both the initial and late excitatory junction potentials were blocked by treatment with atropine (5 x 10^-5 g/ml.).

8. The distribution of inhibitory nerves in the circular muscle cells was investigated. The results indicate that inhibitory nerves arise from Auerbach's plexus situated just beneath the taenia coli and the nerve branches spread over the whole distance from one taenia coli to the next along the ciruclar muscle cells, a width of about 3 mm.

9. The mean conduction velocity of excitation of the inhibitory nerves was 16·0 cm/sec. Hexamethonium, in a concentration of 5 x 10^-6 g/ml. depolarized the circular muscle membrane and lowered the rate of rise and fall of spike, but did not block the generation of inhibitory junction potentials.