Four healthy men were given an insulin tolerance test (0.1 IU/kg BW) six times in different days at the following clock hours: 0400 h, 0800h, 1200 h, 1600 h, 2000 h, and 2400 h, in random order. Four days separated two consecutive tests. The glucose disappearance rate (K value) was calculated for each test between 5 min and 25 min after iv injection of insulin. The statistical evaluation by Cosinor test showed a statistically significant circadian rhythm for the hypoglycemic action of exogenous insulin with a peak at 1030 h (95% condifence limits: 0830 to 1430 h).