1. The chemosensitivity of the surface of parasympathetic neurons in the interatrial septum of the heart of the frog has been explored by focal iontophoretic application of acetylcholine (ACh). 2. There is a specific highly chemosensitive area on the cell surface near synaptic boutons. ACh released close to visually identified boutons produced depolarizing responses which arise more rapidly and are larger than responses evoked at randomly chosen spots on the neuronal surface. 3. Individual synapses, or small numbers of them, can be desensitized by applied ACh without blocking synaptic transmission at nearby synapses on the same neuron. This shows that the method of ACh application resolves chemosensitivity in patches restricted to a few micrometres in diameter.