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1. The action of Ca on membrane excitability and synaptic transmission at motoneurones of the isolated spinal cord of the frog has been studied.

2. Impulse propagation along presynaptic fibres was unaffected by [Ca] in the range of 0-10 mM in the presence of 1 mM-Mg.

3. Motoneurone membrane excitability was within normal ranges when the cord was bathed in low or 0 Ca solutions containing 1 mM-Mg but was depressed in 5 or 10 mM-Ca solutions.

4. Spontaneous miniature synaptic potentials were recorded in the presence of tetrodotoxin. The frequencies were reduced but not abolished in the absence of bath Ca, but were increased greater than twofold in 5 or 10 mM-Ca in comparison to those in normal Ringer solution (1 mM-Ca).

5. Iontophoretic application of Ca both near to and remotely from the intracellular recording site (presumably in the soma) caused facilitation of elicited post-synaptic potentials.

6. It is concluded that the action of Ca at these central synapses is identical to that described at the neuromuscular junction.