Simultaneous recordings of the slow postsynaptic potentials of in vitro rabbit superior cervical ganglion and the muscarinic asynchronous discharge in the postganglionic nerve were obtained in order to examine their relationship. Analyses for linear correlation indicated that the peak amplitude of the S-EPSP cannot fully determine the maximum rate of muscarinic asynchronous discharge. The rate of asynchronous discharge in the postganglionic nerve systematically increased with increases in either the frequency or duration of preganglionic stimulation even after the S-EPSP was at asymptotic levels. We suggest that this deviation from linearity is the result of repetitive spike discharge elicited by the S-EPSP. The S-IPSP had no apparent influence on generation of asynchronous afterdischarge, but may have had an influence on asynchronous discharge occurring during a train of stimuli. Incubation of ganglia with gallamine, an antagonist of the ganglionic muscarinic receptors mediating hyperpolarization, resulted in the selective blockade of the S-IPSP. Neither the S-EPSP nor the muscarinic afterdischarge were suppressed by muscarinic receptor blockade by gallamine, nor did gallamine produce any effect on the nicotinic F-EPSP or the noncholinergic SS-EPSP. Temporary exposure of ganglia to dopamine, in the presence of an inhibitor of catechol-O-methyltransferase, was followed by a potentiation of the muscarinic afterdischarge in accordance with the long-term enhancement reported for the S-EPSP.