(1) Temporary exposure of rabbit's superior cervical ganglion (SCG) to dopamine (DA), in the presence of an inhibitor of catechol-o-methyltransferase (COMT) is consistently followed by a potentiation of the slow (s)-EPSP and s-IPSP, lasting for some hours. The fast (f)-EPSP is not significantly increased, but it is better maintained than in control ganglia. (2) Exposure to the COMT-inhibitor U-0521 alone induces less but substantial potentiations of both s-PSPs. This effect is explained as due to protection of DA released intraganglionically at rest. (3) This evidence suggests that COMT may significantly limit the access of catecholamines to postsynaptic receptors, for at least certain types of neuron-to-neuron synaptic actions. (4) The potentiation of both s-PSPs, whether induced by DA in the presence of U-0521 or by U-0521 alone, is depressed by DA-1 antagonists that have been found to depress DA-stimulation of adenyl cyclase in rabbit SCG; these are spiroperidol, butaclamol and, to a lesser extent, bromocriptine. The specific 'DA-2' antagonists metoclopramide and sulpiride, and the alpha-adrenergic antagonist dihydroergotamine, did not depress potentiation. (5) Potentiation of s-EPSP is viewed as identical in nature to the previously discovered DA-modulatory enhancement of direct muscarinic depolarizing actions (by acetylcholine or its agonists). Potentiation of s-IPSP may be due to a similar DA-modulation of other muscarinic response(s) involved in mediating the s-IPSP. The consistency and comparative ease with which these DA-modulatory effects can be induced, under presently described experimental conditions, should facilitate future study of this mode of synaptic action.