Contractile properties of isolated trabecular meshwork strips have recently been described. In the present paper we characterize the regulation of the outflow pathway in the isolated perfused anterior segment of the bovine eye. Anterior segments of bovine eyes with detached iris, ciliary body and ciliary muscle were perfused at constant pressure of 8.8 mmHg. A constant outflow of approximately 6-8 microliters min-1 could be obtained for at least 3 hr. The calculated outflow resistance was in the range 1.1-1.4 mmHg min microliter-1. The relative outflow was significantly reduced after application of carbachol, reaching a maximal inhibition of 30%. EC50 for carbachol was 3 x 10(-8) mol l-1. Atropin completely blocked the effect of carbachol on outflow. Morphological examination of perfused anterior segments which were perfused with carbachol revealed an intact fine structure of the meshwork cells. Pilocarpine at 10(-5) mol l-1 reduced outflow by 15%. Epinephrine at 10(-5) mol l-1 reduced outflow, while epinephrine at 10(-6) mol l-1 slightly increased the outflow rate. This effect could be blocked by metipranolol. Endothelin-1 in concentrations of 2 x 10(-9) and 2 x 10(-8) mol l-1 inhibited relative outflow by > 30%. Carbachol, pilocarpine, endothelin and a high dose of epinephrine, which have been shown to induce contractions in isolated bovine trabecular meshwork and ciliary muscle strips, induced a reduction of outflow rate and an increase of outflow resistance of the anterior segment. Thus, at least in the bovine eye, the trabecular meshwork per se is directly involved in the regulation of aqueous humor outflow.