The effects of skeletal muscle relaxants on skeletal muscle have been well known, but not effects of those drugs on smooth muscle have intensively studied. In an attempt to study the effects of pancuronium, gallamine, succinylcholine and d-tubocurarine on isolated tracheal preparation of mongrel dog. The writters have performed a set of experiment, of which results can be summarized as follow: 1) Pancuronium in dose of 3ug, 10ugi 30ug, 100ug produced dose-dependent relaxation on contraction. 2) Gallamine in dose of 100ug, 300ug, 1,000ug and 3,000ug produced dose dependent relax histamine action on histamine contraction. 3) Succinylocholine in dose of 10ug, 30ug, 100ug and 300ug produced dose dependent contraction on histamine contraction. 4) d-Tubocurarine in dose of 30ug, 100ug, 300ug and 1,000ug produced dose dependent contraction on histamine contraction. From the results described above, it can be concluded that pancuronium and gallamine have bronchodilating character and succinylcholine and d-Tubocurarine have bronchospastic character.