For improvement of the duration of action of FR35447 (I), 2-(biphenylimino)imidazolidines (III) were synthesized and their hypotensive activity was tested against conscious normotensive rats. 2-(4'-Fluoro-[1,1'-biphenyl]-2-ylimino)imidazolidine (IIIl) exhibited superior hypotensive potency and was comparable to clonidine (II) in its duration of action. The structure-activity relationships of III are also described.