Apraclonidine (para-aminoclonidine) is an alpha agonist that was studied for its effect on the IOP rise following YPC. In a prospective multicentered double-masked study 63 eyes were pretreated with one drop of either 1% apraclonidine or placebo 1 hour prior to performing YAG and again after the laser treatment. The greatest IOP rise in the placebo-treated eyes occurred in the third hour after YPC when the mean IOP rose from a baseline pressure of 16.4 +/- 3.7 mm Hg to 20.8 +/- 6.8 mm Hg (P less than .01). In apraclonidine-treated eyes the IOP fell from a mean of 15.6 +/- 3.8 mm Hg to 12.8 +/- 6.0 mm Hg 3 hours postoperatively (P less than .001). There were five times as many eyes that had a pressure rise greater than 10 mm Hg in the placebo-treated group compared to those treated with apraclonidine. Apraclonidine proved to be highly effective in preventing the rise in IOP following YPC.