The prostaglandin analogue PhXA34 was tested in two studies in normal human eyes; 1, 3, and 10 micrograms of PhXA34 reduced the intraocular pressure by about 2, 3, and 4 mm Hg, respectively, 6 to 10 hours after a single topical dose. The only side effect observed was a slight conjunctival hyperemia after 10 micrograms of PhXA34. In a second study we determined the effect of 10 micrograms of PhXA34 once daily for 7 days on intraocular pressure, outflow facility, aqueous flow, blood-aqueous barrier permeability, ocular discomfort, and hyperemia. The mean intraocular pressure was below 9 mm Hg 12 hours post dose. About one third of the intraocular pressure reduction could be explained by increased outflow facility. Aqueous flow was unaffected. Treatment caused a 21% increase in aqueous fluorescence 1 hour after an oral dose of fluorescein. Mild ocular discomfort and some hyperemia were initially observed in half of the subjects, but frequency and magnitude of these side effects declined during the study.