Objective: To evaluate the possible additivity of the effects of latanoprost and 8-iso prostaglandin E2 (8-iso PGE2) on intraocular pressure (IOP) in monkey eyes with laser-induced glaucoma. Methods: The IOP was measured hourly for 6 hours beginning at 9:30 am on day 1 (baseline day), days 6 and 7 (single-agent therapy), and days 13 and 14 (combination therapy with both agents). Following 1 day of baseline measurement, 4 monkeys with unilateral glaucoma received monotherapy twice daily with either 1 drop of 0.005% latanoprost, or 0.1% 8-iso PGE2, 25 μL, at 9:30 am and 3:30 pm from days 2 through 7. From days 8 through 14, both agents were applied twice daily 5 minutes apart. Results: The maximum reduction of IOP (mean ± SEM) was 8.8 ± 1.9 mm Hg (26%) (P<.05) with latanoprost alone and 6.5 ± 1.0 mm Hg (21%) (P<.01) with 8-iso PGE2 alone, 2 hours after the morning dosing on day 7. A further reduction of IOP of 4.0 ± 0.6 mm Hg was produced when 8-iso PGE2 was added to latanoprost and of 3.0 ± 0.7 mm Hg was produced when latanoprost was added to 8-iso PGE2 on day 13 before the morning dosing. Combination therapy with both agents caused maximum IOP reductions from baseline of 11.3 ± 3.0 mm Hg (33%) (P<.05) (latanoprost with 8-iso PGE2 added) and of 9.8 ± 1.3 mm Hg (31%) (P<.01) (8-iso PGE2 with latanoprost added) on day 14. Conclusion: Latanoprost and 8-iso PGE2 have an additive effect on IOP in glaucomatous monkey eyes. Clinical Relevance: At least 50% of patients are treated with more than 1 ocular hypotensive medication. Thus, the determination of the additive effects on IOP of glaucoma medications will help to define optimum treatment regimens.