A chain drive transfer case is adapted for optional use with various accessory transmissions or rate changers which are interchangeably attached to one side of the chain case. The chain case includes an input shaft which has a splined end projecting through an opening in the case for coupling with the input shaft of any of the accessory transmissions or for use with an engagement or neutral lockout attachment. Each of the accessory transmissions has a tubular output shaft which is concentric with the chain case input shaft and which extends through said opening in the chain case to drivingly engage a chain sprocket which is supported on the chain case input shaft.