1. Laser Doppler velocimetry and computerized image analysis of retinal photographs were used to define retinal vascular autoregulation to 60% oxygen breathing. This normally causes vasoconstriction in the retinal circulation. Normotensive and hypertensive diabetic subjects were studied under conditions of relative normoglycaemia (< 10 mmol) and hyperglycaemia (> 15 mmol) together with control subjects. 2. Retinal blood flow was significantly higher in diabetic subjects when hypertensive and hyperglycaemic than in the same diabetic subjects when normotensive. 3. In the normotensive non-diabetic subjects the oxygen reactivity was 41.16 +/- 14.09%. It was significantly reduced in normotensive 'hyperglycaemic' diabetic subjects (21.75 +/- 15.56%), hypertensive diabetic subjects with controlled blood pressure and 'normoglycaemia' (30.49 +/- 14.20%), hypertensive diabetic subjects with controlled blood pressure and 'hyperglycaemia' (18.36 +/- 11.42%), hypertensive diabetic subjects with uncontrolled blood pressure and 'normoglycaemia' (26.91 +/- 13.43%) and hypertensive diabetic subjects with uncontrolled blood pressure and 'hyperglycaemia' (17.17 +/- 13.24%) (mean +/- SD, all P values < 0.05). 4. In conclusion, retinal vascular reactivity is impaired in diabetic subjects both when they are normotensive and when they are hypertensive. Hyperglycaemia, to a degree commonly encountered in clinical practice, impairs retinal vascular auto-regulation even further.