A system is described for injecting small controlled amounts of fluid, such as de-emulsifier, into a flowing line at high pressure, in a high-pressure environment such as a subsea environment. The system is connected to the flowing line by tubing lines. It is enclosed either in a high-pressure vessel able to withstand high external pressure or in a vessel that is pressure-matched with the environment. The system is able to inject a controlled volume of fluid into the flowing line at relatively low injection rates.