The effects of skeletal muscle relaxants on skeletal muscle have been well known, but not the effects of these drugs on smooth muscle. In and attempt to study the effects of pancuronium, gallamine, succinylcholine, d-Tubocurrarine on siolated mongrel dog tracheal preparations, the writer has performed a set of experiment, of which results can be summarized as follows: 1) Pancuronium in dose of (r/cc) 3,10,30,100 produced dose dependent relaxation on Acetylcholine contraction. 2) Gallamine in dose of (r/cc) 30,100,300,1000 produced dose dependent relaxation on Acetylcholine contracction. 3) Succinylcholine in dose of (r/cc) 30,100,300,1000 produced dose dependent contraction on Acetylcholine contracction. 4) d-Tubocurarine in does of (r/cc) 30,100,300,1000 produced dose dependent contraction on Acetylcholine contracction. From the results described above, it can be concluded that pancuronium and gallamine had bronchodiating character and succinylcholine and d-Tubocurarine had bronchospastic character. But the clinical application of these results are still obscure.