The antimuscarinic potency of new derivatives of phencyclidine containing the adamantyl moiety was evaluated in the guinea pig ileum test. The drugs tested competitively inhibited the acetylcholine-induced contractions. Kd values were calculated and compared to those obtained by direct binding to the muscarinic receptors from mouse brain. The new derivative, |1-(2-phenyl-2-adamantyl) piperidine| was found to be the most potent muscarinic antagonist among the many phencyclidine derivatives known to date. The results are discussed in terms of the acetylcholine-like molecular arrangement of these derivatives that gives rise to a characteristic muscarinic interaction.