Abstract: A partial stroke valve testing apparatus is operably inserted between an emergency shutdown valve and a valve actuator, and comprises a drive cam attached to a drive shaft interconnecting the valve and actuator and an engagement cam attached to an engagement shaft which can be rotated from an operational position to a testing position to bring the engagement cam into operable communication with the drive cam. A key is locked to the engagement shaft to rotate the engagement shaft, and can be removed only when the engagement shaft is in the operational position. A safety stop mechanism prevents the engagement shaft from inadvertently rotating out of the operational and testing positions. A spring-biased pin engages radially-spaced detent seats in the engagement shaft to provide a positive indication of the position of the engagement shaft.