Structure-activity considerations of Nu-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 (imidazolylpheny1) formamidines. It was found that in the guanidine and benzamidine classes, the meta-substituted derivatives (3, 4, 7, and 8) ...