Structure-activity considerations of N alpha-guanylhistamine, the first compound found with detectable H2-antagonist activity, led to the synthesis of a series of conformationally rigid guanylhistamine analogues, namely, (imidazolylphenyl)guanidines, imidazolylbenzamidines, and (imidazolylphenyl)formamidines. It was found that in the guanidine and benzamidine classes, the meta-substituted derivatives (3, 4, 7, and 8) possessed H2-antagonist activity, whereas in the class of formamidines, only the para-substituted derivative 10 was found active. A subsequent increase in the size of the substituent at the formamidino group of 10 led to compounds (15-20) of high H2-antagonist affinity, which was related to the gastric antisecretory effect. Members of this structurally novel class of H2 antagonists were 20- to 50-fold more potent than cimetidine both "in vitro" and "in vivo". Structure-activity relationships are discussed in terms of ionization properties, partitioning behavior, conformational aspects of the selected compound 17, and of possible modes of interaction with the histamine H2 receptor. It was found that the formamidine moiety was an important structural feature and that H2-antagonist activity requires correct steric and electronic properties. Compound 17 (DA 4577), owing to its pharmacological profile and demonstrated safety in animals, was selected to be clinically investigated.