The present investigation was carried out to determine the in vivo effects of garlic and antioxidants on marginally high blood pressure in human subjects as well as the in vitro effects on human endothelial cell (EC) nitric oxide (NO) production. The antioxidant vitamin C alone (2.0 g/d), garlic alone (2.5 g/d), or a combination was administered for 10 days in human subjects with marginally high blood pressure. Furthermore, the effect of garlic extract, garlic ingredients (including alliin, allyl disulfide, diallyl trisulfide), or antioxidants (vitamin C, vitamin E, and vitamins C + E), and combinations on human EC NO formation were examined. Vitamin C alone did not result in any changes in systolic or diastolic blood pressure. In contrast, garlic resulted in a significant lowering (P < .05) of mean systolic but not diastolic blood pressure. In contrast, garlic plus vitamin C resulted in a distinct lowering of mean systolic and diastolic blood pressures to reference ranges. At the cellular level, the various garlic ingredients resulted in significant (P < .01) increase in EC NO production by about 2-fold above control. In contrast, vitamin C, E, or both resulted in slight but not statistically significant increase in EC NO production. However, combinations of the garlic ingredients with antioxidant vitamins resulted in enhanced (P < .01) EC NO production by about 3-fold. The effect of garlic ingredients or garlic ingredients and antioxidants on EC NO production might explain the effects of garlic and vitamin C in lowering marginally high blood pressure.