1. The postjunctional excitatory and inhibitory effects of adrenoceptor and purinoceptor agonists and antagonists were studied in circular smooth muscle strips of cat colon. 2. In the presence of tetrodotoxin (0.5 microM), noradrenaline caused contraction or relaxation of circular smooth muscle at resting tension or with raised tone, respectively. The noradrenaline-evoked contractions were potentiated and the noradrenaline-evoked relaxations were antagonized by propranolol (1 microM), suggesting beta-adrenoceptor involvement. 3. At resting tension, noradrenaline, adrenaline and the selective alpha 1-adrenoceptor agonist, phenylephrine, caused concentration-dependent contractile responses, with EC50 values of 1.8 +/- 0.2 microM, 1.9 +/- 0.4 microM and 4.3 +/- 1.7 microM, respectively. The EC50 values and the amplitude of maximal responses were not significantly different from one another. Clonidine (0.1-500 microM), a selective alpha 2-adrenoceptor agonist, was not effective. 4. Prazosin (0.1-9 microM), competitively antagonized the contractile effects of noradrenaline with an estimated pA2 value of 6.93 and a slope of 1.07 +/- 0.03. The Kb values, estimated from a single shift (0.1 microM prazosin) of the concentration-response curves to noradrenaline, adrenaline and phenylephrine were 92.8 +/- 9.3 nM, 108.7 +/- 6.4 nM and 18.4 +/- 3.1 nM, respectively. 5. At resting tension, adenosine 5' triphosphate (ATP, 5-1000 microM), alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-MeATP, 0.05-50 microM), beta,gamma-methylene adenosine 5'-triphosphate (beta,gamma-MeATP, 0.5-100 microM), and 2-methylthioadenosine 5'-triphosphate (2-MeSATP, 1-500 microM) caused concentration-dependent contractions with EC50 values of 60.5 +/- 15.9 microM, 0.7 +/- 0.1 microM, 7.6 +/- 0.1 microM and 25.3 +/- 12.8 microM, respectively. The maximal responses to alpha, beta-MeATP and beta,gamma-MeATP were greater than maximal responses to 2-MeSATP and ATP.6. Suramin (50-500 microM), competitively antagonized the contractile responses of alpha,beta-MeATP with an estimated pA2 value of 4.92 and a slope of 1.08 +/- 0.04. The Kb values, estimated from a single shift(1I00 microM suramin) of the concentration-response curves to ATP, alpha,beta-MeATP, beta,gamma-MeATP and 2MeSATPwere 52.3 +/- 20.2 microM, 25.2 +/- 4.5 microM, 21.7 +/- 11.0 microM and 11.6 +/- 2.7 microM, respectively.7. At resting tension, reactive blue 2 (100 microM), a selective antagonist of the P2Y-purinoceptor, and 8-(p-sulphophenyl)-theophylline (8-SPT) (1 microM), a selective antagonist of the PI-purinoceptor, did not antagonize the contractile responses to alpha,beta-MeATP (0.5-5 microM). Contractile responses to ATP(50-500 microM) were not altered by 8-SPT (I microM) but were potentiated by reactive blue 2 (100 microM).8. With raised tone, ATP and 2-MeSATP caused a relaxant effect. This effect of ATP was not altered by either tetrodotoxin (TTX) (0.5 microM) or suramin (100 microM), but was antagonized by reactive blue 2(100 microM) and 8-SPT (1 microM), suggesting that inhibitory P2y- and P1-purinoceptors are involved. In contrast, alpha,beta-MeATP and Beta,gamma-MeATP caused only contractions. This contractile effect of alpha,beta-MeATPwas resistant to TTX (0.5 microM) and antagonized by suramin (100 microM).9. In summary, cat colon circular muscle contains postjunctional alpha 1-adrenoceptors and P2X purinoceptors which mediate contractions and P2y- and PI-purinoceptors which mediate relaxation.Postjunctional alpha2-adrenoceptors appear not to be present.