A prospective, randomized study evaluated the effects of unilateral, topical 1% apraclonidine hydrochloride on conjunctival oxygen tension in 10 normal volunteers using a conjunctival oxygen monitor. Conjunctival oxygen tension and intraocular pressure were measured prior to instillation of apraclonidine, then at 1, 3, and 5 hours. Apraclonidine was found to significantly decrease conjunctival oxygen tension 76% at 1 hour compared with baseline values. At 3 hours conjunctival oxygen tension was decreased to 56% and at 5 hours to 10% of baseline. Intraocular pressure was lowered maximally at 3 hours to 40% of baseline measurements in the treated eye. The contralateral eye had a small decrease in conjunctival oxygen tension and intraocular pressure that was not statistically significant. Lid retraction and conjunctival blanching were noted to occur maximally between 1 and 3 hours. Further study is needed to determine if the apraclonidine-induced conjunctival hypoxia noted in this study has clinical significance for ocular blood flow, particularly in patients with glaucoma.