The invention relates to an injection device (1, 100) configured to perform automatic priming upon removal of a protective cap (15, 115) from a cap receiving portion (9, 109) of a housing (2, 102). When mounted on the injection device (1, 100) the cap (15, 115) defines a first position for a drive member (10, 110) relative to the housing (2, 102), which drive member (10, 110) is capable of causing a piston rod (7, 107) to advance a piston (8, 108) in a reservoir (4, 104). Means (13, 120) are provided defining a second position for the drive member (10, 110) relative to the housing (2, 102). During removal of the cap (15, 115) from the cap receiving portion (9, 109) the drive member (10, 110) moves from the first position to the second position under the influence of a spring (11, 111), thereby causing the piston rod (7, 107) to pressurise the reservoir (4, 104).