In recent years, a number of automatic microinjection systems have appeared on the market. These systems replace the simple manual syringe system for forcing the DNA solution out of the microinjection pipet and into the pronucleus of a fertilized one-cell egg. The advantages of such automatic systems are twofold: (1) Because injection is triggered by a foot-operated peddle, the hands are left free to operate the joy-stick controls of the micromanipulators. Since the hands are not constantly moving from one piece of apparatus to another, the process of microinjection is speeded up considerably. (2) Through the application of a low, constant (balance) pressure, DNA solution is flowing out of the holding pipet throughout the injection session. This prevents back-flow of M2 medium into the injection pipet, which would otherwise considerably dilute the DNA solution, and it also prevents blockage of the pipet. Using an automatic injection system, it is found that pipets need not be changed as often as required when using a manually operated system. This chapter describes the operation of an economical injection system supplied by the Narishige company (Tokyo).