A portable pump for infusing liquid substances into a body is provided. The pump has a casing and electromechanical members for converting rotary motion of a motor shaft into linear sliding motion of a pusher for causing movement of a plunger slideable within an interchangeable reservoir in which the substance to be infused is contained in a housing. The housing communicates through an opening formed in a casing wall through which the reservoir can be inserted into and removed from the housing and from which an appendix of the reservoir protrudes for the exit of the substance. The housing has a distal portion with mechanical means for reversible engagement of the reservoir end opposite to the appendix and a proximal portion provided with an annular sealing gasket which surrounds the reservoir and forms a seal between the reservoir and casing wall.
