Binding properties for the muscarinic antagonist quinuclidinyl benzilate (QNB), the nicotinic antagonist tubocurarine and nicotine to crude synaptosomal fractions from mouse and rat brain have been studied. Marked regional differences in number of binding sites for labelled QNB (3H-QNB) were found with a high concentration in the striatum, cortex and a low concentration in the cerebellum. The specific binding sites for labelled tubocurarine (3H-tubocurarine) showed less regional variation. An increased number of muscarinic binding sites (with no change in receptor affinity) was found in rats following long-term treatment with barbital.