An in vitro method is described for the isolation, cannulation, and pressurization of a 10-mm segment of the adult rat testicular subcapsular artery by use of a 200-microns micropipette connected to an adjustable-height reservoir. External and internal arterial diameters were measured by a digital filar micrometer eyepiece calibrated with a Microcode scaler. Transmural pressure was increased stepwise by 20 mm Hg from 20 to 180 mm Hg at 10-min intervals. The following novel triphasic myogenic response was found: a 22.5% increase in lumen cross-sectional area from 20 to 40 mm Hg (p less than 0.05); a 37.3% decrease in lumen cross-sectional area from 40 to 100 mm Hg (p less than 0.05); and a 72.4% increase in lumen cross-sectional area from 100 to 180 mm Hg (p less than 0.05). Papaverine (0.1 mM) completely blocked the transmural pressure-induced vasoconstriction, indicating an active increase in vascular smooth muscle tone by transmural pressure. The calculated blood flow through the testicular subcapsular artery of the adult rat, from measured internal radii at various transmural pressures ranging from 20 to 180 mm Hg by Poiseuille's equation, was constant from 40 to 100 mm Hg and showed a progressive increase from 100 to 180 mm Hg. These data suggest that the myogenic response of the adult rat testicular subcapsular artery may have an important role in the autoregulation of the testicular blood supply.