Abstract The isolated ciliary epithelium contains barium-inhibitable potassium channels. The present study was aimed at testing the in vivo effects of barium on aqueous humor dynamics in rabbits. BaCl 2was administered to one eye by topical delivery or intravitreal injection. Dynamic measurements included intraocular pressure, outflow facility, episcleral venous pressure and aqueous flow (fluorophotometry). Barium dynamics were studied using 133Ba. Intraocular pressure was not altered after topical administration of BaCl 2. 133Ba was not detected in the aqueous after delivery of eyedrops containing the radiochemical. Intraocular pressure decreased following intravitreal injection of BaCl 2(0.15μg). The onset of this pressure reduction was 12 to 16hr, the maximum decrease (−11.3mmHg) occurred at 2 days, and the effect persisted (−4.2mmHg) for approximately 12 days after the injection. Outflow facility and episcleral venous pressure were not altered. However, aqueous humor flow 5 to 6 days after the injection was decreased by 42% to 63% as determined by fluorophotometry or calculated tonographic data. The injection was not associated with findings of intraocular inflammation. Radioactivity was detected in the vitreous within the first 3 days after the injection; however, activity was present in the ciliary body, equally distributed between the cell membrane and soluble fractions, seven days after the injection. Intravitreally injected BaCl 2results in a prolonged intraocular pressure decrease relating to reduced aqueous formation. While the mechanism(s) for the BaCl 2-induced increase in pressure are not clear, possibilities include a Ba 2+interaction with ciliary epithelial K +or N-type Ca 2+channels.