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.