The effects of procaine on the excitatory effects of acetylcholine (ACh) on Aplysia neurones were studied by analysing the current relaxations induced by voltage steps. Procaine was applied by perfusion and ACh was applied iontophoretically under two sets of experimental conditions designed to produce very different ACh concentrations at the membrane. 2. When ACh was applied at high concentration, the current relaxation following a hyperpolarizing step showed two components at procaine concentrations of about 2 X 10(-5) M; a fast increase was followed by a slow decrease. At high procaine concentrations (greater than 10(-4) M), the relaxation has only one component, corresponding to an exponential current decrease. 3. When ACh was applied at low concentrations, the current relaxation usually showed two components. At low procaine concentrations a fast increase was followed by a slower one; at higher procaine concentrations, a fast decrease preceded the slow increase. 4. All the results can be accounted for by assuming that procaine binds preferentially to the activated receptor-channel complex and converts it into a non-conducting complex. The association and dissociation constants were calculated to be (at 12 degrees C, -80 mV) 1.3 X 10(6) M-1 sec-1 and 10 sec-1.
