A preliminary report is given on studies of the retinal circulation in diabetic retinopathy before and after photocoagulation treatment. The local retinal circulation time RCT of selected retinal areas was calculated by means of a fluorescein long-term infusion technique. In 7 eyes with diabetic retinopathy (DR) the mean RCT was significantly higher (2.0 +/- 0.58 sec. range 1.27-2.6) than in 15 control eyes (1.11 +/- 0.34 sec, range 0.50-1.82, Niesel 1976). Assuming an increase of blood flow in DR (L'Esperance 1971, Kohner 1975) the increase of RCT by a factor 2 would imply an increase of the vascular volume by a factor hardly less than 3 indicating the marked impairment of retinal hemodynamics in DR. After photocoagulation, the mean RCT was significantly shortened only in eyes with preoperative RCT values above 2 sec. The reduction of local RCT correlated with the extent of treated retinal areas. The cross-sectional area of the retinal veins measured near the optic disc was generally reduced independently of the extent of coagulation treatment. Taking also the obvious postoperative reduction of the total retinal vascular volume into account, photocoagulation resulted in a marked decrease of blood flow not only in extensively treated but also in areas with little coagulation effects. In DR the disturbed retinal circulation in general tends to normalize following extensive photocoagulation.