Abstract: The levels of plasma ACTH at 2400 and 0800 were determined in 22 patients with simple obesity（obese） and 18 normal weight subjects（normal）. Small dose dexamethasone suppressing test（DST） was done in all obese and 10 normal. Serum cortisol（F） and urinary free cortisol（UFC） were investigated at the same time. The results showed that the level of ACTH at 2400 was the same as that at 0800 in the obese （P＞0.05）, but higher than that the level at 2400 in the normal （p＜0.1）. In fact, the level of ACTH at 2400 in normal was much lower than that at 0800 （p＜0.01）. However, the F values at 2400 and 0800 were similar in both groups, also the levels of UFC were not markedly different. After DST was performed the levels of ACTH, F and UFC were all suppressed. It was suggested that there may be an increase in secretion of ACTH on obese patients with enhanced cortisol metabolism by adipose tissue, while the feed-back regulating function of hypothalamus-pituitary-adrenal axis remains normal.