This study was designed to investigate the effect of Zen meditation on serum nitric oxide activity and oxidative stress (lipid peroxidation). The experimental group included 20 subjects who had practised the Zen meditation programme in a meditation centre located in Seoul, South Korea. The control group included 20 subjects who did not practise any formal stress management technique and were age and sex matched with the experimental group. To provide an assessment of nitric oxide production, the serum level of nitrate/nitrite was determined using Griess reagent. Malondialdehyde concentration was measured as a convenient index of lipid peroxidation by the thiobarbituric acid method. The meditation group showed a significantly higher level of serum nitrate + nitrite concentration and a significantly reduced level of serum malondialdehyde than the control group.