OBJECTIVE To study the effect of a selective alpha 2-adrenoreceptor agonist, brimonidine tartrate, on ocular hemodynamics. METHODS Eighteen patients with ocular hypertension were enrolled in a prospective, randomized, double-masked study in which 0.2% brimonidine tartrate, administered twice daily, was compared with its vehicle in a crossover fashion. The effect on the ocular circulation was assessed by color Doppler ultrasound, which measured blood flow velocities (peak systolic and end diastolic velocities) in the central retinal, ophthalmic, nasal, and temporal ciliary arteries. The following tests were performed at 2 weekly intervals on both treatments, 0.2% brimonidine tartrate and the placebo: intraocular pressure, heart rate, blood pressure, and color Doppler ultrasound. RESULTS Velocities and resistivity indices measured by color Doppler ultrasound in the ophthalmic artery, central retinal artery, nasal artery, and temporal ciliary arteries showed no statistically significant differences between the placebo and 0.2% brimonidine tartrate when compared with baseline values and between the groups. Intraocular pressure was decreased by 17.7% +/- 9.5% with 0.2% brimonidine tartrate (vs 9% +/- 8% with placebo). CONCLUSIONS The hemodynamics of the posterior segment of the eye as measured by color Doppler ultrasound do not appear to be altered by 2% brimonidine tartrate.