In studies on structure-activity relationships of histamine H2-receptor antagonists, lamtidine-analogue derivatives of N-(3-hydroxyphenyl)guanidine and 5- and 7-hydroxy-tetrahydroisoquinoline were prepared and tested for their H2-antagonistic activity on the isolated guinea-pig atrium and on the histamine-stimulated acid secretion of the anaesthetized rat. A further aim of the investigations was to examine the influence of a lengthening of the side-chain as well as the substitution of the aminotriazole with other H2-antagonistic components, on the pharmacological data of the above-mentioned isoquinoline derivatives. All compounds made showed only little H2-antagonistic activity on the guinea-pig atrium. At the acid secretion a noticeable activity especially of the 7-hydroxy-tetrahydroisoquinoline isomers could be observed.