We invesigated whether the circadian rhythms of plasma MHPG and HVA concentrations exist in 11 healthy young adults, and analyzed the patterns of their circadian rhythms. The results were as follows : 1) The change in the mean plasma MHPG concentrations analyzed by repeated measures ANOVA was not statistically significant. Five subjects showed significant circadian rhythms of plasma MHPG concentrations of each individual, and 4 of those had the acrophases between 17PM and 24PM. 2) The change in the mean plasma HVA concentrations analyzed by repeated measures ANOVA was not statistically significant. Six subjects showed significant circadian rhythms of plasma HVA concentrations of each individual, and 4 of those had the acrophases between 21PM and 6AM. In conclusion, the circadian rhythm pattern of plasma HVA concentrations in our result is consistent with the previous study. It was suggested that plasma MHPG and HVA concentrations should be measured more frequently, and the physical activities of subjects be controlled more strictly for the following study.