Previous studies have demonstrated the existence of receptors for gamma-aminobutyric acid (GABA), beta-adrenergic catecholamines and acetylcholine in vertebrate peripheral nerve, and provided functional correlates for activation of both GABA and beta-adrenergic receptors. The present studies show that a cholinergic receptor present on the nerve can also modify impulse pattern. In frog sciatic nerve, both carbamylcholine and dibutyryl cyclic GMP increased the amplitude of the response to the second stimulus of a train at very short interstimulus intervals. The effect of carbamylcholine was blocked by 4-aminopyridine. The results are consistent with the hypothesis that cholinergic agonists mediate an increase in endogenous cyclic guanosine monophosphate (cGMP), which increases the ability of the nerve to follow closely spaced stimuli by inhibiting potassium channels.