Local choroidal flow has been measured using hydrogen (H2) clearance polarography in dog eyes in vivo and in vitro. The accuracy and reproducibility of the method was checked in the eye in vitro by demonstrating a linear relationship between flow measurements obtained by an absolute method and those obtained by the H2 clearance technique. The H2 electrode was placed through a scleral window and its current response to a bolus injection of H2-saturated saline into a proximal artery was measured. The clearance of H2 from the choroidal circulation in vitro was monoexponential and the reproducibility of the measurements at the same site and in the same eye was good. In vivo the technique was used to measure alterations in local choroidal blood flow produced by acute changes of intraocular pressure. A linear relationship between perfusion pressure and choroidal blood flow was obtained. H2 clearance polarography is shown to be an accurate and reproducible method for determining choroidal flow, with the advantage that localized repeated measurements of flow can be made.