Neostigmine injected intra-arterially to the superior cervical ganglion of the cat evoked an asynchronous postganglionic nervous discharge in both normal and denervated superior cervical ganglia. This asynchronous firing was enhanced by tubocurarine but blocked by small doses of atropine. In addition, the responses evoked by acetylcholine in ganglia treated with neostigmine were characterized by two components. The first was blocked by tubocurarine and the second by atropine. Asynchronous firing evoked by repetitive preganglionic nerve stimulation of ganglia treated with neostigmine was blocked by atropine but not tubocurarine. It is suggested that neostigmine (1) has actions on ganglia other than those attributable to inactivation of cholinesterase, (2) may possess both pre- and post-synaptic sites of action, and (3) may unmask a cholinoceptive site which can be blocked by atropine.