The effects of the organic mercurial p-chloromercuribenzene sulfonic acid (PCMBS) on the acetylcholine receptors of two types of neurons from the marine gastropod Anisodoris nobilis were studied. Cell types were distinguished by the ion-specific conductance change resulting from applied cholinergic agents: in certain cells a conductance increase to Cl resulted (Cl-neurons); in other cells a cation conductance increase resulted (D cells). Exposure of the Cl-neurons to 1 mM PCMBS at 4 degrees C had two distinct effects: 1) the sensitivity of the membrane conductance to microelectrophoretically applied carbamylcholine was blocked in a few minutes; and 2) the membrane conductance slowly increased with an inversion potential which was the same as that of the cholinergically induced response. d-Turbocurarine did not appreciably reduce this conductance increase. However, applying carbamylcholine during the exposure period to PCMBS reduced the PCMBS response. The cholinergic response of D cells was affected to a much lesser degree by PCMBS.