In a medium containing 100 mM NaCl and 10 mM MgCl2, the binding of antagonists to rat myocardial muscarinic receptors is well described by the simple Langmuir isotherm. However, in low ionic strength media, while a small proportion of the sites (23%) display a small (2-3 fold) increase in antagonist affinity, the majority demonstrate a substantial (ca. 10-fold) decrease. This reduction is reversed by micromolar concentrations of guanylylimidodiphosphate and other GTP analogues, with consequent abolition of the heterogeneity of the receptor population induced by low ionic strength. Millimolar concentrations of Mg2+ but not other cations strongly potentiate this action of the guanine nucleotides.