An inkjet pen including an ink reservoir for storing ink and providing ink for jetting. A port, located on top of the ink reservoir, fluid-communicated with the ambient air, is used for adjusting the air pressure inside the reservoir. A valve, operated by a spring or a resilient element, normally seals the port, while occasionally opening the port to introduce air into the reservoir when the ink level is low and the underpressure rises. In other embodiments, an elastic bag is included in the reservoir that has an opening communicated with the ambient air through a second port formed on top of the reservoir. The elastic bag expands in response to the increasing underpressure generated in the reservoir when ink is being used. The bag expansion actuates the opening of the valve so as to regulate the underpressure.