Circadian rhythms of platelet activity have been extensively investigated in recent years in view of a possible correlation with the increased morning frequency of thrombotic events. Since a role of enhanced platelet activity has been suggested in diabetic macro- and microvascular disease, the aim of the present study was to analyze the circadian variations of in vitro platelet aggregability in patients with non insulin-dependent diabetes mellitus. Eighteen in-patients, ranging in age from 49 to 73 years, all in good metabolic control, were included in the study. Aggregometric patterns were analyzed in blood samples collected at 0700, 0900, 1100 and 1700. A significant increase of the threshold concentrations for ADP-collagen- and arachidonic acid-induced platelet aggregation has been observed in the interval 0700-1100 or 0700-1700, suggesting, also in diabetic patients as in normal subjects, a maximum of platelet activity in the morning, regardless of postural changes and timing of meals.