Earlier studies on the control of retinal and choroidal blood flow are reviewed and some recent observations on the effects of light on retinal metabolism and retinal and choroidal blood flow in monkeys (Macaca fascicularis) are reported in preliminary form. The retina is nourished by the retinal blood vessels, where blood flow is autoregulated and the choroidal blood vessels where autoregulation is absent. Studies with the deoxyglucose method of Sokoloff indicate that flickering light tends to increase the metabolism of the inner retina, while constant light reduces the metabolism in the outer retina. Retinal blood flow in flickering light, 8 Hz, is higher than in constant light. The sympathetic nerves of the choroid are probably involved in a protective mechanism, preventing overperfusion in fight and flight situations with acute increments in blood pressure. The facial nerve contains parasympathetic vasodilator fibres to the choroid; the physiological significance of these fibres is unknown. The neuropeptides NPY, VIP and PHI are likely to be involved in autonomic reflexes in the eye.