The structural relationship of the competitive histamine H2-receptor antagonist 3-amino-5-(2-amino-4-pyridyl)-1,2,4-triazole (1) to the agonist histamine and to antagonists of the cimetidine type was explored by the design and synthesis of four series of bioisosterically designed prototypes. Biological data from these series was best interpreted as indicating a similarity between the imidazole moiety of histamine and cimetidine and the 2-amino-4-pyridyl moiety of 1. On the basis of this data, sequential replacement of 2-amino-4-pyridyl by 2-[(dimethylamino)methyl]-5-furyl and 2-guanidino-4-triazolyl moieties led to a structurally more potent series of biaryl histamine H2-receptor antagonists. The best of these, 2-methyl-4-(2-guanidino-4-thiazolyl)imidazole (29, CP-57,361-1) was 120 times more potent as a histamine H2-receptor antagonist than 1.