The retinal arterial diameter and its response to changes in perfusion pressure were measured by means of a fundus camera in 65 normal subjects. A reduction of effective MABP was induced by a postural change from a recumbent to an erect position. A small but significant increase in retinal arterial diameter was observed by reduction of the perfusion pressure in all subjects. The reduction of retinal arterial reactivity in response to blood pressure changes (-% change in diameter/delta effective MABP) was significantly correlated with advancing age (p less than 0.01). No significant correlations were observed between the retinal arterial reactivity, magnitude of the retinal arterial diameter (range: 60-140 mu), and systemic blood pressure (range of MABP: 75-110 mmHg). These results indicate that the retinal artery has an autoregulatory mechanism which is influenced by aging.