Effects of reactive red 2 and its parent compound acid red 33 were studied in rat vas deferens and guinea-pig taenia coli. In rat vas deferens, reactive red 2(1 to 10 microM) shifted the concentration-response curve for the P2X-purinoceptor-mediated contraction effect of alpha, beta-methylene ATP slightly to the right and progressively decreased the maximum (apparent antagonist Kd value 0.42 microM). Acid red 33 (1000 microM) shifted the curve to the right without changing the maximum (apparent Kd 386 microM). The concentration-contraction curve of noradrenaline was not altered by reactive red 2. In the carbachol-precontracted guinea-pig taenia coli, reactive red 2 (0.1 to 10 microM) shifted the concentration-response curve for the P2Y-purinoceptor-mediated relaxation effect of adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) progressively to the right; only at the highest concentration of antagonist (10 microM) was the maximum slightly depressed; a pA2 value of 7.55 (Kd 0.028 microM) was derived from the shift. Acid red 33 (1000 microM) shifted the concentration-relaxation curve of ADP beta S to the right without changing the maximum (apparent Kd 171 microM). Reactive red 2 (1 to 10 microM) also shifted the concentration-response curve for the relaxation effect of alpha, beta-methylene ATP, which is mediated by an unclassified P2-purinoceptor, progressively to the right but simultaneously decreased the maximum (apparent Kd 1.6 microM). The concentration-relaxation curve of 2-chloroadenosine was not altered by reactive red 2. Pieces of vas deferens and taenia coli degraded 76 and 66% of added ATP (10 microM) within 30 min, respectively. Reactive red 2(0.1 to 100 microM) progressively reduced this degradation by up to 95%, with IC50 values of 3.9 +/- 0.6 and 3.9 +/- 2.3 microM, respectively. Acid red 33 (1000 microM) reduced the degradation by 30 and 20%, respectively. The results indicate that reactive red 2 is a relatively potent antagonist at both P2X-purinoceptors in rat vas deferens and P2Y-purinoceptors in guinea-pig taenia coli, with a 15 fold selectively for the P2Y-purinoceptor. It inhibits ecto-nucleotidase in both tissues. The dichloro-triazine residue that distinguishes the compound from acid red 33 greatly enhances the potency at both receptor subtypes as well as at the nucleotidase. As regards P2-purinoceptor subtypes, the results confirm the existence of two relaxation-mediating P2-purinoceptors in guinea-pig taenia coli.