The degree of train-of-four (TOF) fade, i.e., the reduction of the fourth to the first twitch height in a train, induced by d-tubocurarine (d-TC) and α-bungarotoxin (α-BX) was investigated. The fade induced by d-TC was pronounced in comparison with that by α-BX, and the difference was analyzed using a kinetic model. Based on the assumptions : (1) Acetylcholine (ACh) binds to the nicotinic receptor and evokes twitch response. (2) The amount of released ACh is dependent on stimulus interval. (3) d-TC interacts competitively with the receptor. (4) α-BX interacts irreversibly with the receptor. It was suggested that the fade by d-TC and α-BX can be explained by the difference of the receptor occupancy by ACh which was caused by different interaction mechanisms of the two muscle relaxants with receptors.