The effect of panretinal photocoagulation on ocular blood flow was studied in pigmented rabbits by means of labeled microsphere impaction and in monkeys by indocyanine-green-dye clearance. In the rabbits, one eye of each animal was photocoagulated and the follow eye of the same animal was used as the control. Twenty-six rabbits received full-scatter panretinal photocoagulation (300 +/- 20 burns, one half burn apart) and an additional 32 rabbits received partial-scatter panretinal photocoagulation (150 +/- 20 burns, one burn apart). After full-scatter photocoagulation there was a significant decrease in choroidal-retinal blood flow measured at 1 to 2 hr, 24 hr, 2 weeks, 4 weeks, and 3 months, but no change in iris-ciliary body blood flow. After partial scatter photocoagulation there was no consistent change. In one monkey, 100 burns were placed around the macular area and in a second monkey 100 burns were placed in the inferior half of the ocular fundus. Choroidal blood flow was significantly reduced in both monkeys for the initial hour during which is was continually measured.