1. Glass microelectrodes were used to record intracellular activity from the striated muscle fibres of the heart of the lobster Homarus americanus.

2. In spontaneously beating hearts bursts of impulses are generated at regular intervals by neurones in the cardiac ganglion. These bursts produce depolarizations of the muscle fibres. Each depolarization is associated with a contraction of the heart. The depolarizations consist of many depolarizing steps; each step is related in a one-to-one manner to a nerve impulse and is believed to be an excitatory junction potential (EJP). The depolarizations (400-700 ms duration in different preparations) exhibit a fast rise to a peak (35-40 mV) followed by a plateau (20-25 mV) which decays to the resting level (51.5 ± 3.2 mV, n = 148).

3. Current-voltage curves indicate that the EJPs do not give rise to regenerative membrane responses.

4. Small, spontaneously produced potentials were recorded in the presence of TTX. Autocovariance tests show that the potentials occur independently and are likely to be miniature junction potentials.

5. Polyneuronal innervation of the muscle fibres was demonstrated by applying stimuli of gradually increasing intensity to the distal ends of cut nerves while recording the responses of a muscle fibre.

6. When a train of stimuli is applied at an intensity to evoke activity from a single axon, the first response of a muscle fibre is usually greater than the second; facilitation of the second and subsequent responses takes place. The degree of facilitation developed depends on the frequency of stimulation. Facilitation decays exponentially with a time constant of a few seconds.

7. In two-pulse experiments the second response was depressed when the interpulse intervals were ≤ 0.5 s.

8. Examples of combined facilitation and depression are presented.

Present address: Laboratory of Neurobiology, Medical Sciences Campus, University of Puerto Rico, San Juan, Puerto Rico 00905.
 Present address: Laboratoire de Neurophysiologie Cellulaire du C.N.R.S., 4 Avenue Gordon-Bennett, Paris i6e, France.