On the takeoff roll the pilot's seat slid back. The aircraft made an abrupt turn to the left, while the pilot was attempting to move the seat back into place. After the unsuccessful attempt was made to move the seat back into place, the pilot retarded the throttle and the aircraft came to a stop after a ground loop. The seats were inspected and found to be stiff and difficult to move fore and aft. The seat rails and latching mechanisms were inspected and no deficiencies were noted.