The effect of an insulin-induced decrease in blood glucose on the retinal blood flow (Q) was studied in 12 type II diabetics, using bidirectional laser Doppler velocimetry (BLDV) and monochromatic fundus photography. Q was first measured during hyperglycemia and then during normoglycemia which was achieved within approximately 3 hours by intravenous insulin administration. At normoglycemia, Q was 15% lower than at hyperglycemia (P less than 0.001). The decrease in Q was larger in patients with shorter disease duration. The authors also determined the regulatory change in Q during 100% oxygen breathing. Although this response remained subnormal, it was improved significantly in normoglycemia (P less than 0.01), particularly in those patients whose blood glucose level was decreased at a slower rate, suggesting that a gradual decrease in blood glucose may be beneficial.