In an attempt to simultaneously detect molecules generated through the metabolism of l-arginine, a high-performance liquid chromatography method with on-line time-controlled preinjection reaction of ortho-phthaldialdehyde derivatization was developed. Plasma concentrations of citrulline, N(G)-hydroxy-l-arginine, N(G)-monomethyl-l-arginine, asymmetric N (G), N (G)-dimethyl-l-arginine, symmetric N (G), N (G')-dimethyl-l-arginine, ornithine, and agmatine were analyzed within 35min, using only 20microl of sample, pretreated by a simple cold ethanol cleanup procedure. Plasma samples of 35 healthy human volunteers were analyzed and results were comparable to other published data. All detection parameters of the method demonstrate that it is a reliable and efficient means for the comprehensive determination of arginine and its metabolites, making this approach suitable for routine clinical applications.