A two-part bioactive agent delivery system, the system including a disposable part comprising an agent reservoir, a bolus chamber, the volume of the bolus chamber being less than the volume of the agent reservoir, an agent outlet, and a valve having a first position communicating the agent reservoir with the bolus chamber and a second position communicating the bolus chamber with the outlet; and a reusable part including a valve driver, a power source and control electronics, the control electronics being adapted to control the valve driver to actuate the valve to deliver bioactive agent from the agent reservoir to the agent outlet; the system further having a spring extending between the agent reservoir piston and a surface of the reusable part or of the disposable part to pressurize the agent reservoir when the spring is compressed and the agent reservoir contains a quantity of bioactive agent.