Blood sugar and IRI were followed for 24 hr in healthy patients, young and adult diabetics, and in subjects with apparently primary obesity on a standard, normal calorie diet. Morning and nocturnal insulin positivity peaks were noted in the normal subjects, though circadians variations in these parameters were mainly related to meals. This periodicity was altered in young diabetics, for whom insulin responsivity was ruled out, and in the adult diabetics, who displayed deficient insulin secretion in the morning, and delayed secretion after the afternoon meal, and in the obese subjects, the quality of whose response was close to that of the previous group. The usefulness of this type on examination in the planning of diet and drug management, quite apart from diagnosis and prognosis, is stressed.