Using high-resolution duplex scanning it is possible to evaluate the blood flow velocity in the central retinal artery of the eye. Four different patient groups were studied with this technique: normals, diabetics with a decreased flow, diabetics with an increased flow and vascular patients with a decreased flow. The eyes of these patients were then treated with topical timolol and the flow measured again. An increase in the flow was found in three of the four groups. This increased flow velocity may be due to a vasodilatory effect of timolol. The results are presented and discussed.