The class of diterpenoids with a 14-carbon cembrane ring, the cembranoids, includes both competitive and noncompetitive inhibitors of the nicotinic acetylcholine receptor (AChR). All 20 coelenterate-derived cembranoids studied in this report inhibited [piperidyl-3,4-3H]-phencyclidine ([3H]-PCP) binding to its high-affinity site on the electric organ AChR, with IC50s ranging from 0.9 microM for methylpseudoplexaurate to 372 microM for lophotoxin. Inhibition was complete with all cembranoids but lophotoxin and most Hill coefficients were close to 1. Methylpseudoplexaurate and [3H]-PCP binding was competitive. Methylpseudoplexaurate and the fourth most potent cembranoid, eunicin, competed with each other for [3H]-PCP displacement, indicating that there exist one or more cembranoid sites on the AChR. Cembranoid affinity for the AChR correlated with hydrophobicity, but was also dependent on other features. Methylpseudoplexaurate and n-octanol also competed with each other for [3H]-PCP displacement, indicating that the cembranoid site is linked to the n-octanol site on the AChR. Unlike lophotoxin, the five cembranoids tested did not inhibit [125I]Tyr54-alpha-bungarotoxin binding to the AChR agonist sites. All seven cembranoids tested on oocyte-expressed electric organ AChR reversibly blocked acetylcholine-induced currents, although the inhibitor concentration curves were shallow and the inhibition was incomplete.