To study the effect of aging on orexin-A, we measured plasma orexin-A concentrations in 82 healthy volunteers (55 men and 27 women) over a wide range of ages (mean 50.2 +/- 13.9 years, ranging from 23 to 79 years). Correlation analyses revealed that plasma orexin-A concentrations correlated with age (r = 0.50, P < 0.01). When comparing three age groups, it appeared that the concentrations in the group of more than 60 years were significantly higher than those in the group of less than 39 years in both genders (P < 0.05). These findings suggest that orexin-A could be involved in aging in a healthy population.