We performed a double-masked, cross-over, dose-response study of apraclonidine hydrochloride (formerly known as ALO 2145) in 20 patients with elevated intraocular pressure (IOP). We administered three concentrations of apraclonidine (0.125%, 0.25%, 0.5%) and vehicle alone bilaterally every 12 hours for one week. Patients were examined 2, 5, and 8 hours after the initial dose, and then on day 2 and day 8. We studied IOP, pupillary diameter, interpalpebral fissure width, blood pressure, and pulse. There was a two-week washout period after each one-week session. All concentrations of apraclonidine significantly lowered IOP. The 0.5% and 0.25% concentrations had equal maximal effects, lowering IOP in each patient by an average of 27% relative to vehicle alone. This corresponded to a mean decrease in IOP of 8.7 mm Hg, from a baseline of 24.9 mm Hg to 16.2 mm Hg. The 0.5% and 0.25% concentrations were significantly more effective than the 0.125% concentration at two and eight hours. Mean interpalpebral fissure width increased in a dose-dependent fashion; the pupillary effect was minimal. Blood pressure and pulse were unchanged. Thirty percent of subjects reported transient dry nose or dry mouth. These symptoms may be dose-dependent.