Ionic current activated by extracellular ATP was characterized using whole-cell voltage clamp of rat sympathetic neurons isolated from superior cervical ganglia. ATP (10–1000 microM) activated an inward current (IATP) at negative holding potentials in almost all the cells (> 97%). The current was reversed near 0 mV in a quasi-physiological external solution, and the concentration dependence could be fitted by a curve with an EC50 value of 60 microM and a Hill coefficient of 1.8. The relationship between IATP and the current activated by ACh (IACh) was examined. ACh (100 microM) activated an inward current one- to fivefold larger than the current activated by 100 microM ATP. IATP and IACh were not additive; the current activated by simultaneous application of ATP and ACh was only as large as the current activated by ACh alone. During current activation by ATP or ACh, the current activated by the other agonist became smaller, suggesting that these agonists reciprocally inhibit their excitatory responses. The reciprocal inhibition appeared to depend on the extent of channel opening because the reduction of the current elicited by each agonist was relieved when the current elicited by the other agonist had been desensitized. Suramin (100 microM), a purinoceptor antagonist, selectively inhibited IATP whereas two open-channel blockers of nicotinic receptor channels, hexamethonium (10–100 microM) and d- tubocurarine (1–10 microM), inhibited IACh without affecting IATP. When 140 mM glucosamine was used as an external cation, only ATP but not ACh activated a considerable inward current at -150 mV. The ATP-induced glucosamine influx was reduced by simultaneous application of ACh. These results suggest that channel activation by ATP and that by ACh are not independent, and these two excitatory neurotransmitters negatively interact with each other at postsynaptic level in rat sympathetic neurons. The interaction between the ATP- and the ACh- induced conductances was hypothetically explained based upon our previous proposal of “channel overlap”; that is, ATP activates a subpopulation of the nicotinic receptor channels.