The binding of several alkyl- and aryl-guanidines andN-hydroxyguanidines to the oxygenase domain of inducible NO-synthase (iNOSoxy) was studied by UV⁄Vis difference spectroscopy. In a very general manner, monosubsti-tuted guanidines exhibited affinities for iNOSoxy that were very close to those of the corresponding N-hydroxyguanidines. The highest affinities were observed for the natural substrates,l-arginine and N x -hydroxy-l-arginine (Kd at thelmlevel).