1. In urinary bladder, M 2-muscarinic receptors predominate, but it is the smaller population of M 3-receptors which mediate detrusor contraction. This study examines the M 2: M 3 ratio and the role of M 2-receptors in contraction of pig urinary bladder. 2. Competition experiments with [ 3H]-QNB determined the ratio of M 2:M 3. In functional studies, affinity values (pK(B)) for 4-DAMP, darifenacin and methoctramine were calculated. Similar experiments were performed on tissues following selective M 3-inactivation (incubation with 40 nM 4-DAMP mustard in the presence of 1 μM methoctramine to protect M 2-receptors), precontraction with 50 mM KCl and relaxation with isoprenaline (30 μM) or forskolin (1 μM). 3. In competition binding, displacement of [ 3H]-QNB by 4-DAMP, darifenacin and methoctramine best fitted a two-site model suggesting a predominant (70-80%) population of M 2-receptors. 4. On normal detrusor in vitro, 4-DAMP and methoctramine caused surmountable antagonism of responses to carbachol with pK(B) values of 9.37±0.07 and 6.05±0.05 respectively. Darifenacin caused unsurmountable antagonism, the apparent pK(B) value being 8.61±0.10. 5. In tissues where the M 3-receptors had been inactivated and cyclic AMP levels elevated, 4-DAMP and darifenacin were less potent, with apparent pK(B) values of 8.72±0.08 and 6.74±0.07. In contrast, methoctramine was more potent, the apparent pK(B) value increasing significantly to 6.86±0.06. 6. These data suggest that the pig bladder possesses a similar muscarinic receptor population to the human bladder and that the M 3-receptor subtype mediates contraction of the normal detrusor muscle. However an involvement of M 2-receptors in contraction can be observed following pharmacological manipulation of the receptor population.