Plasma angiotensin II concentrations were measured in 14 patients in diabetic ketoacidosis and in two patients with hyperosmolar non-ketotic hyperglycemia, before treatment and again when blood glucose control was restored. In the ketoacidosis group plasma angiotensin II before treatment was markedly raised in all patients with otherwise uncomplicated diabetes, but was within the normal range in some patients with long-term complications such as neuropathy, retinopathy and nephropathy. Mean angiotensin II before treatment was significantly higher in otherwise uncomplicated patients than in those with long-term complications. However, plasma angiotensin II decreased with improved control in all. Angiotensin II levels did not correlate with indices of rehydration such as changes in blood urea, packed cell volume and calculated changes in plasma volume. There was, however, a significant negative association between pre-treatment angiotensin II and pH. Two patients with hyperosmolar non-ketotic hyperglycemia were more dehydrated but less acidotic. Pre-treatment angiotensin II in each was well below the mean of the ketoacidosis group. These data are further evidence that the renin-angiotensin system may be imparied in diabetics with long-term complications. In addition, they suggest that factors other than fluid depletion are also important in activating the renin-angiotensin system in uncontrolled diabetes.