An improved surgical stapling instrument is provided for applying surgical staples to human tissue. The instrument has an articulated stapling head assembly mounted by a rotatable and flexible support shaft on an actuator handle assembly with an improved actuator mechanism for closing and firing the stapling head assembly. The stapling head assembly comprises a fixed jaw with a staple cartridge and a movable jaw with an anvil for clamping the tissue therebetween. The actuator mechanism comprises a jaw closure lever for closing the jaws and a staple firing lever for actuating a staple driver to drive the staples from the staple cartridge into the tissue and against the anvil. The actuator mechanism also includes a cam pulley mounted for rotation by the staple firing lever and secured to a firing cable for actuating the staple driver. A set of drive gears rotatably positioned on opposite sides of the cam pulley mesh with gear sectors on the staple firing lever to rotate the cam pulley and apply tension to the firing cable when the staple firing lever is actuated. The cam pulley has a contoured cam lobe for actuating the firing cable with different mechanical advantages as the pulley is rotated. An anti-backup member engages the cam pulley to prevent the pay-out of the firing cable before and after the staple firing lever is actuated. The length of the support shaft can be adjusted to set the tension in the closure cable so that the jaws are completely closed by the jaw closure lever to produce staples of uniform height when the staple firing lever is actuated.