L-NG-NITRO arginine p-nitroanilide (L-NAPNA), L-NGnitro arginine methyl ester (L-NAME) and L-NG-monomethyl arginine (L-NMMA) inhibit rat cerebellar nitric oxide synthase (NOS) with IC50s of 1.4 ± 0.1 μM, 0.81 ± 0.16 μM and 5.1 ± 0.07 μM respectively. L-NAPNA inhibits the late phase of formalin-induced hindpaw licking (ED50,57.2 mg kg-1) and acetic acid induced abdominal constrictions (ED50, 25 mg kg-1) in the mouse. L-NAPNA is approximately 65 times less active than L-NAME as an inhibitor of endothelium-dependent relaxation in the rabbit aorta and about 10 fold less potent as a vasopressor in the anaesthetized mouse. LNAPNA shows some degree of selectivity for the central NOS isoform and may be of clinical interest for the treatment of pain.