We investigated the modulatory role of endogenous adenosine on tetanic-induced (50 Hz for 5 s) nicotinic facilitation of [3H]acetylcholine release (5 Hz for 50 s) from rat motoneurons. Adenosine deaminase (0.5 U/ml) and the adenosine A(2A) receptor antagonist, 3,7-dimethyl-1-propargyl xanthine (DMPX, 30 microM), facilitated post-tetanic [3H]acetylcholine release. Release inhibition caused by tubocurarine (1 microM), dihydro-beta-erythroidine (1 microM) and alpha-conotoxin MII (0.1 microM) was attenuated after tetanic preconditioning. Nicotinic inhibitory action was fully restored after adenosine A(2A) receptor block by DMPX or adenosine deaminase. DMPX (10 microM) caused a leftward shift of the inhibitory dose-response curves for d-tubocurarine (0.1-1 microM), dihydro-beta-erythroidine (0.03-10 microM) and alpha-conotoxin MII (1-300 nM) on post-tetanic twitch amplitude. In contrast, the post-tetanic twitch depression caused by alpha-bungarotoxin (3-100 nM, which had no effect on transmitter release) was attenuated by DMPX (10 microM). It is concluded that activation of adenosine A(2A) receptors by endogenously generated adenosine prevents the post-tetanic release facilitation mediated by nicotinic alpha3beta2 autoreceptors.