Conformationally restricted analogues of the endogenous NOS substrate L-arginine and the arginine based NOS inhibitors NG-methyl-L-arginine (L-NMA) and NS-iminoethyl-L-ornithine (L-NIt) were synthesized for evaluation as inhibitors of human NOS. Incorporation of a phenyl ring into the C4-C5 backbone chain provided 2-aminophenylalanine analogues which retained potent NOS inhibition. Structurally related analogues of 3-aminophenylalanine were significantly weaker inhibitors. © 1997 Elsevier Science Ltd.