A series of 3-acylbenzamidine (amidino)hydrazones 7a-h, the corresponding (hetero)aromatic congeners 7i-p, and 3,3'-bis-amidino-biaryls 25a-e were synthesized. The hydrazones 7a-p were prepared by conversion of the corresponding acyl nitriles 1a,c-d,i,n-p to the imido esters 3a,c-d,i and the amidines 5a,c-d,h-i, followed by a reaction with aminoguanidine, or vice versa. Similarly, the biaryl 3,3'-dinitriles 23a-e were converted, via the imino esters 24a-c or the imino thioesters 27d-e, to the diamidines 25a-e. These new products are conformationally constrained analogues of methylglyoxal bis(guanylhydrazone) (MGBG). They are up to 100 times more potent as inhibitors of rat liver S-adenosylmethionine decarboxylase (SMDC) and generally less potent inhibitors of rat small intestine diamine oxidase (DAO) than MGBG. Some of these SAMDC inhibitors, e.g., compounds 7a, 7e, 7i, 25a, and 25d, have shown antiproliferative effects against T24 human bladder carcinoma cells. These products, whose structure-activity relationships are discussed, are of interest as potential anticancer agents and drugs for the treatment of protozoal and Pneumocystis carinii infections.