We tested the hypothesis that cyclooxygenases (COX) or COX products inhibit nitric oxide (NO) synthesis and thereby mask potential effects of NO on reactive hyperemia in the cutaneous circulation. We performed laser Doppler flowmetry (LDF) with intradermal microdialysis in 12 healthy volunteers aged 19-25 years. LDF was expressed as percent Cutaneous Vascular Conduction (%CVC) or as maximum %CVC (%CVC_max) where CVC is LDF/mean arterial pressure. We tested the effects of the non-isoform specific NO synthase inhibitor nitro-L-arginine (NLA, 10mM), the nonspecific cyclooxygenase inhibitor ketorolac (Keto, 10 mM), combined NLA+Keto and NLA+sodium nitroprusside (SNP, 28 mM) on baseline and reactive hyperemia flow parameters. We also examined the effects of isoproterenol (Iso) a -adrenergic agonist that causes prostaglandin independent vasodilation to correct for the increase in baseline flow caused by Keto. When delivered directly into the intradermal space, Keto greatly augments all aspects of the laser Doppler flow response to reactive hyperemia: peak reactive hyperemic flow increased from 41±5 to 77±7%CVC_max, time to peak flow increased from 17±3 to 56±24 sec, the area under the reactive hyperemic curve increased from 1417±326 to 3376±876 %CVC_max-sec, and the time constant for the decay of peak flow increased from 100±23 to 821±311 sec. NLA greatly attenuates the Keto response despite exerting no effects on baseline LDF or on reactive hyperemia when given alone. Low dose NLA+SNP duplicates the Keto response. Isoproterenol increased baseline and peak reactive flow. These results suggest that cyclooxygenase inhibition unmasks nitric oxide dependence of reactive hyperemia in human cutaneous circulation.