Rabbit atria were isolated with the extrinsic right sympathetic and vagus nerves attached and perfused with Tyrode solution. Acetylcholine overflow was determined after labelling of the transmitter stores with [14C]choline and fractionation of the radioactivity on cation exchange columns. Sympathetic nerve stimulation (SNS, 2 Hz, 3 min) carried out together with vagus nerve stimulation (VNS, 2 Hz, 3 min), but each SNS pulse preceding a vagal one by 19 ms, caused a facilitation of acetylcholine overflow of about 60% versus independent controls in the absence of SNS. Antagonists of putative neurotransmitters were tested to find out the prejunctional mediator involved in the facilitation. The facilitation was not significantly reduced by prazosin, rauwolscine, idazoxan, or propranolol, excluding mediation by alpha- or beta-adrenoceptors. However, guanethidine abolished evoked noradrenaline release and facilitation, suggesting that it is due to a compound co-released with noradrenaline from postganglionic noradrenergic nerves. Pretreatment of rabbits with reserpine which reduced noradrenaline content of atria and SNS evoked overflow by 94% did not affect the facilitation of acetylcholine release which, due to the cardiostimulatory action of SNS being absent, resulted in enhanced depression of atrial force. We conclude that the facilitation is due to release of a reserpine-resistant co-transmitter from sympathetic nerves. Possible mediation of the facilitation by ATP through P2X- or P2Y-purinoceptors was excluded by ineffectiveness of alpha, beta-methylene ATP-preperfusion, of suramin and cibacron blue, respectively. However, the selective A2 adenosine receptor antagonist CP 66,713 reduced the facilitation by 25% whereas DPCPX (A1-selective) had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)