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β-Adrenoceptors have been demonstrated in the urinary bladders of many animals including the guinea pig. However, there is little information on the subtypes involved in the antispasmodic activity of β-adrenoceptor activation in the guinea-pig detrusor. The present study uses the non-selective β-agonist isoproterenol, the antagonist nadolol, the β₂-selective agonists salbutamol and terbutaline, the antagonist ICI 118551, and the β₁-selective antagonist metoprolol, to demonstrate functionally the subtypes existing in the guinea-pig detrusor. Isoproterenol dose-dependently reduces the myogenic activity in the guinea-pig detrusor induced by mild depolarization with 20 mM potassium in the tissue bath. At the supramaximal concentration of 30 µM, isoproterenol achieves 73 ± 2% of the reference maximal response. This activity of isoproterenol is reduced to 9 ± 5, 24 ± 6 and 54 ± 1 % in the total blockade of β, β₁ and β₂ with nadolol, metoprolol and ICI 118551, respectively. Consistently, salbutamol and terbutaline at the same concentration produce only 35 ± 1 and 38 ± 4% of the response, respectively. Thus, both β₁- and β₂-adrenoceptors are present in the detrusor of the guinea-pig urinary bladder. Although activation of either subtype results in antispasmodic action, the larger portion of the antispasmodic activity appears to be associated with the activation of the β₁-subtype.