Abstract: A cannula (20) useful for endoscopic surgery. The cannula includes an inner tube (44) in which an inner tube (46) is removably seated. A surgical instrument (24) can be positioned at the surgical site through the inner tube and irrigation fluid can be flowed to the surgical site through the inner tube. The tubes are spaced from each other to form a channel through which fluid can be selectively withdrawn from the surgical site. Fluid flow to and from the surgical site through cannula is controlled by a valve assembly (50) that has a single valve cock (188). The valve cock can be positioned to allow maximum irrigation flow/no suction flow or no irrigation flow/maximum suction flow. Bores (198, 204) internal to the valve cock are shaped so that, while the suction flow is initially incrementally increased, the irrigation flow is at constant level.