A variable flow rate injector provides accurate, precise, and reproducible injection volumes that have low dispersion. The invention is particularly well-suited for HPLC injection volumes 500 nL but can be used to inject larger volumes and in different applications as well. Injections are performed at a first flow rate and separations are performed at a second flow rate. For improved HPLC system performance, the first flow rate is less than the second flow rate. The injector uses a variable flow rate fluid supply that allows rapid switching between flow rates desired for injection and flow rates desired for separations.