Primary Raynaud’s (PR) disease is a disorder of the cutaneous blood vessels, particularly of the fingers and toes such that they show vasospasm in response to cold or emotional stress. Hyper-reactivity of the sympathetic nervous system or a ‘local fault’ at the level of the blood vessels, have been suggested to account for PR (Belch, 1990). However, the underlying mechanisms are still not clear. In previous studies, we proposed that vasoconstrictor cyclo-oxygenase (COX) products limit dilator responses to acetylcholine (ACh) in pre- and post-menopausal women with PR disease (Easter & Marshall, 2005). Notably, in women with PR, COX inhibition with aspirin potentiated responses evoked in the finger by iontophoretic application of ACh. We now hypothesise that in PR patients, the COX pathway is distorted such that it produces vasoconstrictor COX products that include O₂^– (Taddei et al. 1998). Thus, we have used Vitamin C to investigate the possible involvement of O₂^–, in limiting finger dilatation in PR disease. In 7 pre-menopausal women with PR disease and 7 age-matched healthy control women, cutaneous red cell flux (RCF) was recorded from the dorsal surface of 2 different fingers of the left hand before and during microiontophoresis of ACh and the NO (nitric oxide) donor sodium nitroprusside (SNP) during the early follicular phase of their menstrual cycle. ACh or SNP was applied iontophoretically with: (7 pulses of 0.1mA for 20s each followed by 1 pulse of 0.2mA for 20s, at 60s intervals) and (5 pulses of 0.1mA for 20s, followed by 1 pulse of 0.2nA for 20s at 120s intervals), respectively (Hendry & Marshall, 2004). This was repeated 3h after oral administration of the antioxidant vitamin C (1000mg) and then again, 30min after aspirin (600mg). All values are expressed as mean ± S.E.M. The pulses of ACh produced incremental increases in RCF up to 204 ± 28 PU (perfusion units) in women with PR disease. Vitamin C potentiated the responses (269 ± 45 PU; P <0.05, paired t test) but aspirin had no further effect (231 ± 26 PU). By contrast, SNP evoked increases in RCF (to 85 ± 9 PU) that were not affected by Vitamin C (77 ± 11) or aspirin (82 ± 9). In control women, ACh evoked increases in RCF (to 190 ± 33) or SNP (to 88 ± 15) that were not affected by Vitamin C (178 ± 28; 86 ± 13) or aspirin (183 ± 23; 77 ± 10), respectively. These results support our hypothesis that in PR patients, cutaneous vasodilatation evoked in the finger by the endothelium-dependent dilator ACh is limited by O₂^–, produced by the COX pathway.