INTRODUCTION: Argon laser trabeculoplasty (ALT) is an intraocular pressure lowering method that is overall safe and powerful, but often complicated by transient postoperative intraocular pressure rises. In prevention of this complication, we frequently use two potent alpha-adrenergic agonists--brimonidine and apraclonidine. OBJECTIVE: The aim of this study was to compare brimonidine 0.2% and apraclonidine 0.5% efficacy and safety in prevention of intraocular pressure elevation after ALT. METHODS: This was a prospective, randomized, double-masked and comparative study. This study included 27 POAG patients, 15 received 0.2% brimonidine, and 12 received 0.5% apraclonidine before laser surgery (22 eyes in both groups). Intraocular pressure readings were taken 1, 2, 3, 24 hours and 7 days after ALT. Student's t-test was used to analyse data between two groups, and chi2 test to compare data within groups. Value p less than 0.05 was considered statistically significant. RESULTS: We found statistically significantly lower IOP in eyes that received 0.2% brimonidine at readings taken 1 hour after ALT (p = 0.001). There were no statistically significant differences in other IOP readings between two groups. CONCLUSION: A single preoperative drop of brimonidine 0.2% had similar efficacy and safety as apraclonidine 0.5% in preventing transient IOP elevations after ALT.