OBJECTIVE To determine the effect of bumetanide inhibition of Na-K-Cl cotransport on aqueous humor formation and outflow facility in living cynomolgus monkeys, outflow facility in organ-cultured human eyes, and contraction in bovine ciliary muscle and trabecular meshwork strips, in vitro. METHODS Aqueous humor formation in monkeys was measured fluorophotometrically for 6 hours, 1 to 6 weeks before, immediately after, and 2 to 6 weeks after bumetanide was administered intravitreally (final concentration approximately 100 or 500 microM) or intravenously (0.01 or 0.03 mg/ kg at 0 and 3 hours). Outflow facility in monkeys was determined by two-level, constant-pressure perfusion of the anterior chamber for 45 to 60 minutes before and after bumetanide was administered by bolus intracameral injection (100 microM, initial anterior chamber concentration) or by exchanging the anterior chamber with 2 ml 10, 100, or 500 microM bumetanide. Urine volume was measured 3 hours after administration of intravenous bumetanide in various diluents. The effect on intraocular pressure in organ-cultured human eyes was determined for 48 hours by constant-flow-variable-pressure perfusion with 10 microM bumetanide. Contraction of fresh bovine ciliary muscle and trabecular meshwork was measured isometrically with a force-length transducer system after exposure to 100 microM bumetanide +/-1 microM carbachol. RESULTS The bumetanide concentrations used had little effect on outflow facility or on aqueous humor formation in normal monkeys, on intraocular pressure in organ-cultured human eyes, or on contraction of bovine ciliary muscle and trabecular meshwork strips. Intravenous bumetanide increased urine volume, regardless of the diluent used. CONCLUSIONS These results suggest that Na-K-Cl cotransport is not involved functionally in regulation of aqueous humor inflow and outflow and in contractility of ciliary muscle and trabecular meshwork.