Nine days after medial septal lesion a 20% increase in the number of muscarinic antagonist binding sites in rat hippocampus was observed without any change in the affinity for agonist or antagonist. Chronic atropine treatment (s.c. 5 mg/kg, twice a day for 14 days, 20 mg/kg once a day for 14 days or 100 mg/kg for 4 days and 20 mg/kg for 10 days, once daily) led to an increase in the number of muscarinic antagonist binding sites in rat hippocampus with 35, 80 and 80% respectively and also lowered the affinity for 3H-antagonists in a dose dependent manner. Agonist binding studies also indicated an increase in receptor number and a decrease in affinity. The latter change can possibly be explained by the presence of residual atropine 24 h after the last injection. If this is taken into account we may conclude that muscarinic supersensitivity evoked either by severing the input or by chronic pharmacologic blockade both produced "new receptors' with ligand binding properties similar to the original receptors.