Rats treated chronically with atropine or atropine and neostigmine showed marked alterations in the responsiveness of receptors at the neuromuscular junction. Receptors in the phrenic nerve-hemidiaphragm interacting with d-tubocurarine were unaffected by atropine but exhibited a supersensitivity after neostigmine administration. Succinylcholine-sensitive sites developed a subsensitivity with both atropine and neostigmine treatment. Acetylcholine-induced contractures in the denervated hemidiaphragm showed a hypersensitivity after both drug administrations, but responses to carbamylcholine were unaffected. It is suggested that there exists a heterogeneous population of receptors at the mammalian neuromuscular junction who can be identified by their responses to the effects of chronic drug application.