An apparatus for receiving and positioning a medical instrument relative to the body of a patient when the instrument is positioned partially within the body or completely outside the body. The apparatus comprises a housing and a motor assembly positioned within the housing. The motor assembly includes a motor and a first medical instrument engaging member operatively connected to the motor and moveable relative to the housing in response to the operation of the motor. A second medical instrument engaging member cooperates with the first medical instrument engaging member to drive the medical instrument within the housing when the medical instrument is positioned within the housing between the first and second medical instrument engaging members. The apparatus can form a portion of a trocar, a structure secured to a trocar or a structure that is free of a connection to a trocar or other medical instrument support.