The main objective was to evaluate a Scanning Laser Ophthalmoscope (SLO) based particle tracking method as a means of quantitative assessment of retinal blood velocity and vessel diameter changes in response to hypoxia and hyperoxia. Retinal blood velocities were measured by tracking fluorescent microspheres (1.0 microm diameter) in anesthetized adult pigmented rats. Velocities were calculated based on microsphere position changes and the recording frame rate. Hypoxia was induced by inspiring a mixture of nitrogen and air and hyperoxia was induced by inspiring 100% oxygen. Average blood velocities during hypoxia obtained for arteries, veins, and small vessels (diameter < 40 microm) were 39.9 +/- 9.9, 34.9 +/- 2.7, and 8.8 +/- 1.8 mm/sec, respectively, whereas during hyperoxia, the average blood velocities obtained were 23.7 +/- 6.2, 28.2 +/- 2.7, and 7.6 +/- 0.7 mm/sec. Hypoxia was found to increase the diameters of arteries by 25% but did not change the diameters of veins; whereas, hyperoxia was found to decrease their diameters by 25% and 18%. Changes detected in vessel diameter and blood velocity suggest that the level of oxygen tension alters retinal hemodynamics. Dynamics of retinal hemodynamics in response to hypoxia and hyperoxia can be assessed using the SLO imaging method.