In quiescent preparations of guinea-pig right atria the action of ACh applied in the superfusion medium or released from parasympathetic nerve fibres was investigated by membrane potential measurements. 2. ACh-containing solutions induced hyperpolarizations which did not show desensitization. 3. The relationship between hyperpolarization amplitude (corrected for non-linear summation) and ACh concentration could formally be described by simple saturation kinetics with an apparent dissociation constant KACh = 1.3 x 10(-6) M. 4. Stimulation with impulses subthreshold for myocardial excitation induced a membrane hyperpolarization by releasing ACh from post-ganglionic parasympathetic nerve fibres. 5. The hyperpolarization reached maximum about 1 sec after the onset of stimulation. This slow development of hyperpolarization cannot be accounted for simply by diffusion from the site of release to the receptor site. 6. The hyperpolarization declined exponentially with a time constant of about 3 sec. In the presence of neostigmine (3 x 10(-6) M) the hyperpolarization lasted for 2 min or more. It is concluded that the action of ACh is primarily terminated by enzymic hydrolysis.
