1 The binding of eleven agonists to muscarinic receptors in the rat heart has been measured in competition with [3H]-N-methylscopolamine. 2 Full analysis of binding required the resolution of three components (SH, H and L). 3 The proportion of the H component was independent of agonist structure. The proportion of the SH component ranged from 2-36% of the total and was dependent on the agonist. The proportion of the L component varied in a complementary way from 59-22% of the total. 4 The ratios of the affinities of ten of the agonists to the three components of the receptors were constant; the weakest binding agonist, choline, had lower ratios of affinity. 5 When saturated with guanylylimidodiphosphate (GppNHp) the SH receptor population was no longer detectable and the H receptor population was reduced by 63%: about 85% of the receptors was in the L population. 6 The affinity constants of agonists for the H and L forms of the receptors were not changed by GppNHp. 7 The results are interpreted in terms of the effects of accessory proteins on the proportions of binding and non-binding conformations of the receptor.