The guinea-pig taenia caeci contains both P1 and P2 purinoceptors mediating relaxation. The P2 purinoceptors have been further characterized using an experimental approach designed to minimise complicating factors. In the presence of the adenosine uptake inhibitor S-(4-nitrobenzyl)-6-thioinosine (NBTI, 300 nM) and a pA100 concentration of the P1 purinoceptor antagonist 8-sulphophenyltheophylline (140 microM), the potency order of agonists was: 2-methylthio-ATP >> adenosine 5'-triphosphate (ATP) = alpha, beta-methylene ATP > beta, gamma-methylene ATP >> uridine 5'-triphosphate. Suramin antagonized ATP (pA2 = 5.52 +/- 0.17, Schild plot slope = 0.67 +/- 0.08) and 2-methylthio-ATP (pA2 = 5.78 +/- 0.30, Schild plot slope = 1.37 +/- 0.39) while responses to 5'-N-ethylcarboxamidoadenosine (NECA) were unaffected. The findings suggest that suramin, while it is selective for P2 relative to P1 purinoceptors, is not a true competitive antagonist. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) antagonized ATP in isolated guinea-pig vas deferens, but had no effect on responses to ATP in guinea-pig taenia caeci indicating it is selective for P2X relative to P2Y purinoceptors.