A convenient simple model for a high velocity fluid line, connecting a hydraulic actuator to a 4/3 zero lapped valve, is presented. The conditions under which the proposed model is valid, are deduced. The system mathema-tical model, in case of constant supply pressure operation and pure inertial external load, shows that the actuator response to step opening of the valve depends on two parameters; the system dimensionless inertial resistance and its dimensionless capacitance. The former equals the ratio between the fluid line inertial resistance and that of the load, while the latter accounts for the system capacitance, valve resistance, load inertial resistance, and the supply pressure. Numerical solution of the system governing equations verifies that at small values of the dimensionless capacitance, cavitation occurs at certain values of the dimensionless inertial resistance, a phenomenon that can not be predicted when the fluid line inertial resistance is ignored. At small dimensionless capacitances, the fluid line inertial resistance is shown to have considerable effect on the transient response, and can be made use of to improve the system performance. Ale combination of dimensionless capacitance and inertial resistance, at which minimum settling time and reasonable values of overshoot and delay and rise times occur, is determined.