ABSTRACTTopical instillations of 1.0, 10, and 20 μg/50 μl of prostaglandin PGA2, 0.5 and 1.0 μg/50 μl of PGA2isopropyl ester, and 0.5, 1.0, 5.0 and 10.0μg/50μl of PGF2αisopropyl ester were evaluated in the normal dogs and glaucomatous beagles eyes. Each concentration of drug was evaluated for a seven day period. On Day 1 baseline values were obtained, days 2-4, the drug was instilled (once a day) and on days 5-7 post-treatment values were measured. All concentrations of PGA2failed to lower intraocular pressure (IOP) in the normal and the glaucomatous (P>0.72) dogs. PGA2isopropyl ester decreased IOP in the normal dogs and in the glaucomatous beagles (P<0.01). The declines in IOP were significant at ½ to 1 hour and continued for up to 5 hours. No significant change in IOP occurred in the non-treated fellow eye of the normotensive dog (P<0.54) and the glaucomatous beagle (P<0.29). All concentrations of PGF2αisopropyl ester significantly decreased IOP in the treated eyes of the normotensive dog (P<0.05) and the glaucomatous beagle (P<0.01). The significant change in IOP occurred within one hour after the instillation of PGF2αisopropyl ester. The IOP remained lower than the baseline pressures 24 hours post-treatment for both the normotensive and glaucomatous dogs. Maximal change in IOP for normal dogs was a decrease of 9 mm Hg while the glaucomatous beagle had a decrease of 19 mm Hg. No significant change in IOP occurred in the non-treated fellow eye of the normotensive animal (P<0.16) and the glaucomatous beagle (P<0.40). The side effects of PGF2αisopropyl ester were miosis and mild conjunctival