The plasmatic levels of TSH in 2 group of diabetic patients (7 decompensated and 8 decompensated, but in treatment) were measured at 9:30 and 23:30 hours. The mean glycemia levels were of 280 +/- 45 and 150 +/- 30 mg/dl (p less than 0.0005). There was no significant difference between daily TSH and nocturnal TSH in any of the groups, but there was a tendency for the nocturnal TSH to be higher in decompensated patients. There was no difference when comparing the TSH of the first group to the TSH of the 2nd group. The mean TSH N/TSH D was superior by 1 (1.36 in decompensated and 1,095 in treated patients). The correlation between glycemia and TSH D was negligible in all groups. The data suggests the tendency that the circadian rhythm of TSH in maintained in diabetes decompensation with shorter rhythm registered in treated patients. This shows a certain normality in the suprahypophysary area in charge of the rhythm and is similar to the minor liberation of TSH after TRH stimulus that other authors have described as happening in the decompensation of diabetes mellitus.