Membranes isolated from bovine atria were labeled with [3H]quinuclidinyl benzylate (3H-QNB), in control conditions and after 0.02% Triton X-100. This treatment inactivated about 20% of muscarinic receptor sites without loss of protein. The remaining 80% sites showed no changes in affinity, as determined by equilibrium or kinetic binding. Competition experiments with carbachol showed no differences in IC50 and Hill number between the control and detergent-membranes, suggesting that the different populations of agonist binding sites are inactivated in equal proportions by the detergent. In binding experiments, done in the presence of carbachol and guanine nucleotides, the detergent treated membranes were slightly more sensitive to the enhancing action of the nucleotide. The inhibition caused by ammonium ions was also more marked in the Triton X-100 treated membranes. The decay of binding with thermal inactivation was faster in the detergent treated membranes and this effect was enhanced in the presence of ammonium ions. These results may be interpreted as an indication that the receptors, remaining after the mild Triton X-100 treatment, are equally sensitive to the inactivation. We suggest that, while maintaining the heterogeneity of sites, the detergent produces a perturbation that could affect the molecular interactions between the receptor and other components of the membrane.