INTRODUCTIONThe efficient delivery of exogenous DNA to cells for expression and function studies is an essential technique of modern cell biology, and direct delivery of genetic material by microinjection remains a reliable means of transfection. In the system described here a constant flow of sample is delivered from the tip of the pipette, and the amount of sample injected into the cell is determined by how long the pipette remains in the cell. The system can be assembled at relatively low cost using commercially available components: Typically, a pressure regulator that can be adjusted for two pressures (back pressure and injection pressure), a capillary holder, and coarse and fine micromanipulators. A constant-flow system, by design, injects less reproducible cell volumes than does a pulsed-flow system. However, with practice, this difference can be made negligible.