An apparatus delivers a dosage of a chemical stimuli or drug in such a controlled manner that the concentration at the point of delivery is known at each and every point in time. The apparatus includes a plurality of vessels with each vessel containing a different concentration of the drug or chemical stimuli. A valve has a plurality of inlets and a single outlet wherein each inlet is connected to a corresponding vessel. An improved culture chamber is fluidly connected to the outlet of the valve and is used to develop the desired concentration. A first pump is positioned preferably between the valve and the chamber to provide a transport force for delivering the chemical stimuli or drug to the chamber. A suction tube is positioned proximate the top of the chamber and has a bottom open end which defines the level of liquid within the chamber. The suction tube thereby keeps the volume within the chamber constant. A second pump provides a transport force for removing excess liquid from the chamber. The valve, the first pump and the second pump are controlled by a computer control system. The computer control system determines which inlet of the valve is fluidly connected to the outlet to provide a preselected concentration of the chemical substance to the chamber with the pump being controlled by the computer to provide a predetermined flow rate of the medium to the chamber. The second pump is controlled by the computer removing excess volume from the chamber to insure that the volume within the chamber is kept constant. The control of the valve, the first and second pumps, are such that the concentration of the chemical stimuli or drug in the chamber is controlled and known at each and every point in time.