In this review, we discuss the regulation of ocular blood flow, the role it may play in disease, and the physiologic methods used to assess it. Altered perfusion may play a role in several ocular diseases. In particular, derangements in ocular blood flow have been offered as possible factors in the development of glaucomatous optic neuropathy. In fact, recent studies have suggested that glaucomatous eyes may lack the degree of circulatory autoreg-ulation found in healthy eyes. The autoregulation of ocular blood flow is in response to tissue pressure, myogenic changes, and metabolic levels. To assess the eye's vascular autoregulation, physiologic perturbation of these mechanisms during hemodynamic assessment provides an autoregulatory "stress test." Along these lines, we have refined the technique of altering arterial levels of oxygen and carbon dioxide, thus provoking a metabolic autoregulatory response, during measures of blood velocities in ocular vessels. Using somewhat similar methods, others have documented changes in ocular hemodynamics by altering blood gas levels. Results of our own work suggest that retinal blood flow may be regulated to maintain constant oxygen delivery. Likewise, our studies using carbon dioxide suggest that retinal blood flow may be steeply dependant on Pco2, for these results to be placed in their proper context, future studies directed at better defining normal autoregulation are needed.