Diaminoguanidine (DAG) and NG-amino-L-arginine each produced a time- and concentration-dependent inactivation of the citrulline-forming activity of all three NOS isoforms. DAG inactivates both the NADPH-oxidase and the citrulline-forming activities of GH3 pituitary nNOS while NG-amino-L-arginine inactivates only its citrulline-forming activity. The inactivation by DAG of GH3 nNOS NADPH-oxidase and citrulline forming activities is stimulated by (6R)-5,6,7,8-tetrahydrobiopterin (BH4) cofactor, follows pseudo-first-order kinetics and is not substrate saturable. DAG-induced inactivation of the citrulline-forming activity for the iNOS and eNOS isoforms displayed maximal inactivation rates of 0.37 and 0.14 min-1 and Ki values of 385 and 670 microM, respectively. At 1 mM DAG and saturating BH4, half-times of inactivation of 0.7, 8, and 2 min were observed for the nNOS, eNOS, and iNOS isoforms, respectively. NG-Amino-L-arginine-induced inactivation of the citrulline-forming activity of the nNOS, iNOS, and eNOS isoforms displayed maximal inactivation rates of 0.35, 0.26, and 0.53 min-1 and Ki values of 0.3, 3, and 2.5 microM, respectively. The inactivation of the NOS activities by both DAG and NG-amino-L-arginine in preincubations required the presence of oxygen and Ca2+, consistent with an inactivation mechanism that requires active metabolism by NOS. Methylguanidine and 1,1-dimethylguanidine exhibited a reversible inhibition pattern in contrast to all three NOS isoforms. Neither agent exhibited significant isoform selectivity.