Since the retinal vessels have no sympathetic innervation, blood flow in response to raised blood pressure is dependent on autoregulation. To determine the effect of hypertension on retinal haemodynamics and the autoregulatory capacity of the retinal circulation under conditions of normoglycaemia and hyperglycaemia, retinal blood flow was measured before and after raising the systemic blood pressure in ten normal control subjects, ten diabetic subjects with blood glucose < 10 mmol l-1 and ten diabetic subjects with blood glucose > 15 mmol l-1. A controlled rise in systemic blood pressure was achieved using an intravenous infusion of tyramine. Retinal volume flow was determined from red cell velocity using laser Doppler velocimetry and from retinal vessel diameter measurements using digital image analysis of fundus photographs. With a 40% increase in mean arterial blood pressure (MAP), there was a significant increase in retinal blood flow of 32.9 +/- 7.1% in non-diabetic controls. In diabetics at the low blood glucose level, the increase in retinal blood flow was significant at 30% increase in MAP (23.6 +/- 8.7%, P = 0.032) and at 40% increase (49.9 +/- 12.03%, P = 0.004). Diabetics with high blood glucose failed to autoregulate at any of the increased levels of MAP (15% increase, 27.0 +/- 11.1%; 30% increase, 66.9 +/- 19.8%; and 40% increase, 101.9 +/- 21.4%; P < 0.022). The coefficients of autoregulation showed that in non-diabetic controls, retinal vascular autoregulation broke down with increases in MAP of between 30 and 40%. In diabetic subjects, it broke down between 15 and 30% in normoglycaemia and at less than 15% in hyperglycaemia. This study demonstrates an impairment in retinal vascular autoregulation in response to raised systemic blood pressure in diabetic subjects, more so at an elevated blood glucose level, thus providing a mechanism for the detrimental effect of hypertension on diabetic retinopathy.