The effect of guanidine on transmitter release in the ciliary ganglion of the chick.

1. The effect of guanidine hydrochloride on synaptic transmission at the chick ciliary ganglion was studied. 2. In bathing solution containing low calcium concentrations, guanidine produced a dose-dependent increase in the number of quanta released by the presynaptic action potential. No post-synaptic effects were observed. 3. Intracellular presynaptic recordings and monitoring of the presynaptic action potential post-synaptically via the coupling potential showed that the action of guanidine was independent of any alterations in the presynaptic spike.