1. In the pleural ganglion of Aplysia californica, groups of cells were identified that respond differently to an iontophoretic injection of ACh. The anterior neurones are excited by ACh, whereas the medial cells are inhibited. The inhibitory response is biphasic, consisting of a short-latency, rapid component and a longer-latency, slow component. Homologous responses (e.p.s.p.s in the anterior cells and a two-component inhibitory response in the medial cells) are evoked in these same cell groups by stimulation of an identifiable presynaptic neurone.2. The e.p.s.p. and the corresponding ACh potential are completely eliminated by hexamethonium which has no effect on either of the inhibitory potentials. Both the e.p.s.p. and the rapid i.p.s.p. (and the corresponding ACh potentials) are blocked by tubocurarine, dihydro-beta-erythroidine, strychnine and brucine. These drugs have no effect on the slow inhibitory potential, whether elicited synaptically or by ACh injection. The slow response can be selectively blocked by methylxylocholine, tetraethylammonium (TEA), and phenyltrimethylammonium (PTMA). Since the three types of potentials were found to be differentially affected by ACh antagonists, it was concluded that the various responses are due to activation of three different ACh receptors.3. Of the cholinomimetics tested, only carbamylcholine imitates all three actions of ACh. Nicotinic agents, which were shown to activate the two curare-sensitive receptors, have no stimulating effect on the curare-insensitive receptor. This latter receptor can be selectively stimulated by arecoline. The cholinomimetics were shown to have a secondary blocking effect on the receptor(s) they stimulate.4. Muscarine, even at high doses, is ineffective as either a stimulating or a blocking agent on any of the three receptor types. The muscarinelike drugs oxotremorine, methacholine, and pilocarpine have only weak and non-specific cholinomimetic action on these receptors. Their blocking effects are likewise negligible.5. The two curare-sensitive receptors, which are presumably the same as those described by Tauc & Gerschenfeld (1962), respond like vertebrate nicotinic receptors to both cholinomimetics and cholinolytics. The third receptor type, on the other hand, has a unique pharmacological profile. It is unaffected by both nicotine and muscarine, and is blocked neither by curare nor by atropine. Knowing that it can be stimulated by arecoline and blocked by methylxylocholine, TEA and PTMA does not facilitate its incorporation into the classical scheme of cholinergic receptors.