The importance of the renin-angiotensin system in the eye has not been clearly established, nor is it known whether it is of importance in the evolution of diabetic retinopathy. Circulating renin is produced in the kidney and other tissues; its main importance is in the control of blood pressure. Elevated levels have been found in diabetic patients with microvascular complications. There is also some suggestion that the prorenin content is elevated in diabetics and that this is an indicator of the development of microvascular complications in the next few years. Angiotensin-converting enzyme (ACE) inhibitors have been found to reduce the leakage in early diabetic retinopathy, but it is possible that this was only due to a reduction of blood pressure. Angiotensin II was found to induce neovascularisation in corneal pocket experiments, and the replication of aortic endothelial cells has been reported. We were unable to find any increased replication of retinal endothelial cells treated with angiotensin II. It is possible that the main function of the renin-angiotensin system in the eye is the regulation of local blood flow.