Recently, an isomeric mixture of herbal anti-inflammatory naphthoquinones shikonin and alkannin, and their derivatives, have been found to impair cellular responses involving nitric oxide (NO) and NO synthesis, like the acetylcholine-induced relaxation response of rat thoracic aorta and NO release from murine RAW 264.7 macrophages. However, the mechanisms of such effects, including whether NO synthase (NOS) activity is affected, remained unclear. We herein investigate possible targets of shikonin in these NOS-related events. Shikonin by itself dose-dependently inhibited the rat thoracic aorta relaxation in response to acetylcholine (pD'(2) value: 6.29). Its optical enantiomer, alkannin, was equally inhibitory in the aorta relaxation-response assay. In RAW 264.7 cells, shikonin inhibited the lipopolysaccharide-induced NO production by 82% at 1 microM. A cell-free assay to verify direct effects on NOS activity showed that shikonin inhibits all isoforms of NOS (IC(50)s, 4 - 7 microM), suggesting NOS as an inhibition target in both the events. Further possible targets of shikonin that might be involved in the inhibitions of the acetylcholine-induced aorta relaxation response and the NO generation by RAW 264.7 cells are also discussed. It is shown for the first time that shikonin inhibits NOS activity.