A method for injecting a chemical, such as an odorant, from a chemical supply into a fluid-containing system such as a natural gas or an LPG pipeline. A tank of odorant is maintained at a pressure above ambient, but below pipeline pressure. An injection conduit communicates the odorant tank with the pipeline. A hydraulic pressure booster is located in the injection conduit for pressurizing the chemical to a pressure above that of the pipeline. Flow-control apparatus located within the injection conduit for metering chemical to be injected into the pipeline is either (a) drip-style metering valve adjustable between a drop-wise setting and a steady-flow setting or (b) a pair of valves one of which is a flow valve allowing larger volumes to be injected and the other which is a drop-wise flow valve for metering smaller volumes of the chemical.