Some herbal medicines, including Aconiti tuber (Aconitum carmichaeli Debeaux, Ranunculaceae), have been recognized as being effective for the treatment of a "peripheral uncomfortable feeling of cold (hie)". We hypothesized that these compounds affect peripheral vascular function via the nitric oxide (NO) system, which leads to recovery from "hie". To answer this question, we investigated Aconiti tuber-induced changes in plasma levels of nitrite (p-NO2-) and nitrate (p-NO3-), final nitric oxide-oxidation products measurable in vivo. After written informed consent was obtained, patients suffering from "hie" were treated with several kinds of kampo (Japanese traditional herbal medicine), selected on the basis of traditional theory. Twenty-four patients took kampo formulas, some included Aconiti tuber (n=11; A-group) and others did not (n=13; C-group), for 4 weeks. p-NO2- and p-NO3- levels were measured before the start and after 4 weeks of treatment. In the A-group, the p-NO2- plus p-NO3- (p-NOx) level was significantly increased at 4 weeks (p=0.04), while that of the C-group was not. There was a statistically significant increase in the p-NOx level of the A-group as compared to the C-group (d.f.=1,22, F=9.38, p=0.006). The results suggest that Aconiti tuber may increase NO production in humans.