WE report here the crystal structures of two muscarinic antagonists of acetylcholine, (−)-(S)-hyoscine hydrobromide1 and (−)-(S)-hyoscyamine hydrobromide (atropine). Correlation of these two structures and that of quinuclidinyl benzilate hydrobromide2 shows marked similarities among the three molecules although they are subjected to very different crystal packing forces. This suggests that intramolecular forces impose a preferred conformation on many such antagonist molecules. The structural correlations may be used to derive a set of rules for predicting the conformation relevant to muscarinic antagonism of acetylcholine for many molecules which at first sight are rather dissimilar to atropine (T. J. P., in preparation).