A method is reported in which the isolated bovine eye is perfused through a long posterior ciliary artery with buffered physiological saline, to provide simultaneous monitoring of drug effects on intraocular pressure (IOP), vascular resistance and the condition of the blood-aqueous barrier.With perfusion under constant pressure of 45 mm Hg, perfusate flows at 1.64±0.12 tnl.min−1(mean±SEM) and IOP is 7.26±0.16 mm Hg. Applying a constant flow rate of 2.25 ml.min−1, IOP averages 10.19±0.32 mm Hg and in both cases this can be maintained for around 2h. Increasing the perfusion flow rate from 1.5 to 3.5 ml.min−1produces a 76% rise in perfusion pressure but IOP increases only insignificantly (<10%). The inclusion in the perfusion fluid of dextran and albumin to maintain oncotic pressure similar to that of plasma makes no difference to the IOP achieved and does not affect the leakiness of the barrier. The preparation shows a net consumption of oxygen, supporting the hypothesis that the aqueous humour formed is secreted by active transport processes.Timolol (in bolus doses of 1–300 nmol) injected into the perfusing fluid is shown to induce a dose-dependent fall in IOP within 5 min, reaching a steady state within 40 min. Timolol, however, causes no significant change in vascular resistance, whether this is measured as perfusion flow rate under constant pressure or as perfusion pressure at constant flow rate, nor does it alter the permeability of the barrier. Other beta-blockers such as oxprenolol and betaxolol also induce dose-dependent decreases in IOP.By applying a fluorescein dilution technique, it is found that the aqueous formation rate (Kout= 0.0046 min−1, or 12.9μ1.min−1) is also reduced by timolol and, in a dose-dependent manner, by the new carbonic anhydrase inhibitor, MK-927.The bovine perfused eye offers a useful method for studying the mechanisms of action of drugs on IOP and aqueous humour formation, in isolation from the complicating influences of the CNS and the cardiovascular system and without the necessity to kill animals for experimental purposes.