A portable, ambulatory infusion pump for use with standard pre-filled infusion medicament containers is powered by a miniature compressed gas cylinder coupled to a mechanical pressure regulator which supplies regulated gas at a substantially constant pressure to an inflatable bladder. The bladder is positioned between a substantially rigid pressure surface and a moveable pressure transfer plate and pushes against the transfer plate when inflated. A pre-filled medicament container is positioned over the pressure transfer plate and is squeezed between the transfer plate and the pump housing cover as the plate is displaced in response to inflation of the flexible bladder. A safety interlock assembly is connected between the pressure regulator and the inflatable bladder and prevents inflation of the bladder unless the housing cover is completely closed. Upon inadvertent opening of the pump, the internal pressurization system is vented to atmosphere.