Purpose: To simultaneously quantify the magnitude and response characteristics of retinal arteriolar diameter and blood velocity induced by an isocapnic hyperoxic provocation in a group of clinically normal subjects. Methods: The sample comprised 10 subjects (mean age 25 years; range 21-40 yrs). Subjects initially breathed air for 5 to 10 minutes followed by oxygen (O₂) for 20 minutes, and then air for a final 10 minute period using a sequential re-breathing circuit (Hi-Ox SR, Viasys) to maintain isocapnia. Retinal arteriolar diameter and blood velocity measurements were simultaneously acquired with the Canon Laser Blood Flowmeter (CLBF-100). The magnitude of the response, the response time and response lag of diameter and velocity were calculated. Results: In response to hyperoxic provocation, retinal diameter was reduced from control values (mean ± SD) of 111.6 ± 13.1µm to 99.8 ± 10.6µm (p<0.001) and recovered after withdrawal of hyperoxia. Retinal blood velocity and flow concomitantly declined from control values of 32.2 ± 6.4mm/sec and 9.4 ± 2.5µL/min to 20.7 ± 3.4mm/sec and 5.1 ± 1.3µL/min, respectively (p<0.001 for both velocity and flow), and recovered after withdrawal of hyperoxia. The response time and response lag were not significantly different for each parameter between effect and recovery. The response time and response lag were not significantly different between diameter and velocity. Conclusion: Arteriolar retinal vascular reactivity to hyperoxic provocation is rapid with a maximal vasoconstrictive effect occurring within a maximum of 4 minutes. Although there was a trend for diameter to respond before velocity to the isocapnic hyperoxic provocation, the response characteristics were not significantly different between diameter and velocity.