The antagonistic property of ethyl p-(4-ethoxycarbonyl-4-phenyl-l-piperidinoethyl)fumaranilate (5) was investigated. Compound 5 was found to antagonize morphine analgesia in a complex manner which could not be described as a simple competitive or noncompetitive type. The antagonism, however, lasted for over 6 hr suggesting that 5 has a high affinity for the analgesic receptors. Compound 5 appeared to possess dependence liability in the single-dose suppression test. In the electrically stimulated isolated guinea pig ileum, 5 acted like an agonist. No antagonistic activity of 5 was apparent in the latter two tests.