Circadian variations in blood glucose, plasma insulin and human growth hormone response were studied in six healthy males who received 100 gm. oral glucose loads at 6 a.m., noon, 6 p.m., and midnight. The tests were conducted at seven day intervals, and each was preceded by a ten hour fast. During the three days before each test the subjects received meals containing no less than 300 gm. carbohydrate per day. Blood samples were drawn at 0, 15, 30, 60, 90, 120, and 180 minutes. A clear circadian variation occurred in the blood glucose levels, with lower values in the morning and higher values at 6 p.m. and midnight. The insulin profiles showed a trend toward lower afternoon and night values, with a noon peak. The afternoon insulin-glucose ratios were significantly lower. HGH values were inconsistent and tended toward higher afternoon and night basal levels. The results confirm the existence of a circadian variation in the blood glucose response to oral glucose loads in healthy men. This might in turn result from a circadian variation in the insulin response, probably secondary to changes in the pancreatic β cell sensitivity to glucose. This basic mechanism is believed to sustain the conditioning influence of other hormones, HGH being one of them.