1 The present study was undertaken to investigate the interaction of the renin ± angiotensin system (RAS), bradykinin and the sympathetic nervous system with cholinergic transmission in the rat airways. Experiments were performed on epithelium-intact and epithelium-denuded preparations of rat isolated trachea which had been incubated with [ 3 H]-choline to incorporate [ 3 H]-acetylcholine into the cholinergic transmitter stores. Tracheal preparations were subjected to electrical ®eld stimulation (trains of 1 ms pulses, 5 Hz, 15 V) and the stimulation-induced (S-I) e ux taken as an index of transmitter acetylcholine release. 2 In both epithelium-intact and epithelium-denuded tracheal preparations, the a 2 -adrenoceptor agonist UK14304 (0.1 and 1 mM) inhibited the S-I e ux, in a concentration-dependent manner. The inhibition of S-I e ux produced by UK14304 (1 mM) was antagonized by the selective a 2 -adrenoceptor antagonist idazoxan (0.3 mM). Idazoxan (0.3 mM) alone had no e ect on the S-I e ux. 3 Angiotensin II (0.1 and 1 mM) was without e ect on the S-I e ux in either epithelium-intact or epithelium-denuded tracheal preparations. When angiotensin-converting enzyme was inhibited by perindoprilat (10 mM), angiotensin II (1 mM) was also without e ect on the S-I e ux. Similarly, in the presence of idazoxan (0.3 mM), to block prejunctional a 2 -adrenoceptors, angiotensin II (0.1 and 1 mM) did not alter the S-I e ux. When added alone, perindoprilat (10 mM) did not alter the S-I e ux. 4 In epithelium-denuded preparations, bradykinin (0.01 ± 1 mM) inhibited the S-I e ux. In epitheliumintact preparations, there was also a tendency for bradykinin (0.1 and 1 mM) to inhibit the S-I e ux but this was not statistically signi®cant. However, when angiotensin-converting enzyme and neutral endopeptidase were inhibited by perindoprilat (10 mM) and phosphoramidon (1 mM), respectively, bradykinin (1 mM) signi®cantly inhibited the S-I e ux in epithelium-intact preparations as well as in epithelium-denuded preparations. The inhibition of the S-I e ux produced by bradykinin, in the combined presence of perindoprilat (10 mM) and phosphoramidon (1 mM), was una ected by the additional presence of the cyclo-oxygenase inhibitor indomethacin (10 mM) and/or the nitric oxide synthase inhibitor N G -nitro-L-arginine (100 mM), in either epithelium-intact or epithelium-denuded preparations. 5 In conclusion, the ®ndings of the present study suggest that airway parasympathetic nerves are endowed with a 2 -adrenoceptors which subserve inhibition of transmitter acetylcholine release. Under the present conditions, however, transmitter acetylcholine release is not subject to transneuronal modulation by noradrenaline released from adjacent sympathetic nerves in the airways. Moreover, angiotensin II and perindoprilat do not appear to modulate acetylcholine release from parasympathetic nerves of the airways. In contrast, bradykinin inhibits acetylcholine release from airway parasympathetic nerves but this action of bradykinin is limited by the activity of...