The possible involvement of purines in the non-adrenergic non-cholinergic (NANC) relaxation of rat duodenum was studied using an isometric-isovolumic preparation. Purines (adenosine, AMP, ADP, ATP) induced a concentration-dependent, tetrodotoxin (TTX)-insensitive, fall in both endoluminal pressure and isometric tension. The relaxation induced by adenosine and by 2-chloroadenosine was selectively antagonized by 8-phenyltheophylline (1, 10 nM, 0.5 microM) and the ATP-induced relaxation was opposed by alpha, beta-methylene ATP (10 microM) and by reactive blue 2 (10 microM). Electrical field stimulation (EFS) caused TTX-sensitive inhibitory effects similar to those induced by ATP. None of the purinergic antagonists used were capable of affecting the EFS-induced relaxation. Our results indicate that both P1 and P2 purinoreceptors are present in muscle of the rat duodenum and are not involved in the NANC relaxation.