Twelve members of a new series of 1-trialkylmethyl-1-6, 7-dimethoxy-3, 4-dihydroisoquinoline (D type) and eleven analogs of 1-trialkylmethyl-6, 7-dimethoxy isoquinoline (I type) were investigated for their pharmacological properties, with special reference to the toxicity and spasmolytic activity, comparing with papaverine. Along these isoquinoline, 1-methylisoquinoline, 6, 7-dimethoxyisoquinoline and their respective 3, 4-hydrogenated derivatives, were also examined for the elucidation of structure-activity relationship. All the compounds of D type produced a marked inhibitory action on the isolated rabbit intestine. The most potent members were n-tripropyl, ethyl-n-propyl-n-butyl and methyl-ethyl-n-butyl derivatives. Their spasmolytic actions on both barium and histamine contraction of the intestine exceeded those of papaverine, though the effect on acetylcholine spasm was slightly inferior. They were less toxic in mice than in the latter. The intestine inhibitory activity and toxicity of I Type compounds were both smaller than those of D type analogs. The differences between the two types seemed to be related to the solubility. Other smaller components of the papaverine structure, namely, the primitive isoquinoline derivatives were more toxic, but much less spasmolytic than papaverine. In mice toxic doses of all the compounds caused tremors, excitability and convulsions followed by a generalized central depression. Respiratory paralysis may be direct cause of death. Toxic symptoms were generally similar to those in frogs. The stimulant properties were increased by the saturation of the 3, 4-position on the isoquinoline nucleus.