1. The effect of calcium on synaptic transmission has been studied by intracellular recording of pre- and post-synaptic potential changes in the stellate ganglion of the squid.2. For a given presynaptic ;input' (propagated spike, or local depolarizing pulse after tetrodotoxin treatment), the post-synaptic response increases with external calcium concentration [Ca](o) in a highly non-linear fashion, indicating that transmitter output varies with more than the second power of [Ca](o) over a certain concentration range.