Retinal blood flow was studied in 9 normal volunteers and 36 diabetic patients. The method used was based on the measurement of the mean transit time of flourescein in the superior temporal quandrant of the retina and on estimation of the vascular volume by measuring vessels diameters. The results showed that patients with mild or no retinopathy had significantly increased volume flow compared with normals, those with moderate retinopathy had a slight but not significant increase and those with severe retinopathy had blood flow similar to that found in normals. The mean transit time was reduced significantly in those with mild or no retinopathy, but was similar to normals in those with moderate and severe retinopathy. Following succesful pituitary ablation and photocoagulation retinal blood flow was reduced compared with pre-treatment studies.