1 The affinity of 17 compounds for muscarine-sensitive acetylcholine receptors in atrial pacemaker cells and ileum of the guinea-pig has been measured at 29 degrees C in Ringer-Locke solution. Measurements were also made at 37 degrees C with 7 of them. 2 Some of the compounds had much higher affinity for the receptors in the ileum than for those in the atria. For the most selective compound, 4-diphenylacetoxy-N-methylpiperidine methiodide, the difference was approximately 20-fold. The receptors in the atria are therefore different the structure from those in the ileum. 3 The effect of temperature on affinity are not the same for all the compounds, tested indicating different enthalpies and entropies of adsorption and accounting for some of the difficulty experienced in predicting the affinity of new compounds.